(note: I do not consider "Dex" Clerics to be "real" Clerics. They would most likely be better on PvP, but such character is designed to play solo, not to support parties)

What you need to be Support/Cleric?

0. To select yourself, just click your name in upper left corner. That's when you want to buff or heal yourself.

P A T I E N C E. This is not class for someone, who wants to enter huge pack of mobs, kill them all in 10 seconds, drink a potion and move further. This is not class for people, who like to solo everything. It is not for people who want to get rich quick. You will party almost all the time, thus amount of loot you get will be lower - make sure that, if you party with stranger, items are set on "orderly", otherwise you will get nothing. With friends, you can split it afterwards, so no big deal.

Most of time is spent on watching party members HP bars and their buffs. In smaller parties, it might seem like easy job, just buff 1-2 people and sit to regain mana, healing occasionaly. Bigger ones - this is where problems start. Keeping 7 people fully buffed is very tiring, especially once you are Cleric. Muse has 5 buffs top, 2 of which are AOE. Cleric has 11, with 5 AOE(3 of which last *very* short). Clerics(not *muse*, but 2nd job already) are in high demand, but don't try to party with 6 ppl, you will only tire yourself and rewards will not be comparable to effort.

1. Stats.

Most important thing is INTelligence, improving your buffs(except for Damage Support) and healing spells by roughly 1%/3.15 INT. Now, how much INT to have is dependant on you. Some people get just INT and nothing else. Some add Dex to avoid some hits(although Blessing Armor is usually better). Another good choice is to get some STR to carry sloted shield with good topaz *and* sloted wand, thus giving you even more INT(g7 topaz in shield would be +35, around 11% better buffs).
In short, get INT every level, maybe adding 1 point to STR or DEX if you have spare points. It will be quite a while before you have good topaz to insert into shield(putting +5 there is a waste), so STR might wait. Minimum STR for socketed shield(round shield) is 36, so that would be enough(unless you use +STR items, which isn't really recommended). Also, SEN increases damage for Wands, but it's effects are minimal, critical rate increase gives more damage than +atk from it.
For PvP, that would getting mostly STR as secondary stat, both for extra HP and defence, as well as damage. Every little bit helps, since it might let you live *just* one second longer, which will be enough to cure yourself.

There is not clear % setting here, only thing is that INT must be *much* higher than any other stat.

2. Items.

Wand or Staff - that's old debate. For many levels, Staff is much superior. Higher atk, higher attack speed, higher MP decrease. Even though Wands are ranged, they are slow and have lower damage - not that it matters, since you probably won't fight directly anyway. However, once you get socketed shield, INT bonus from extra topaz is superior to Staffs MP decrease bonus.
For PvP - it's Staff for Staff Stun, unless you're against class you *don't want to meelee with - say Knight. Than just use Wands and hit&run.

Muse shields(books etc.) - these are useless... Really. Only advantage they have is INT requirements instead of STR, meaning you can use them much earlier than normal shields. However, they don't have slots, which is main reason for carrying shield in the first place. Supposedly they can get slots if crafted by Artisan with *very* high Sen, but I have yet to see such item.

Other equipment - two options you should look for. First, +INT. Quite simple, more INT = better support, which is what you're aiming for. In theory, you could wear Visitor Look +6 INT entire game Second good option is 5%MP decrease / +2 INT. Slightly less int than previous option, but 5% MP can be quite a bit with high level buffs - these cost quite a bit and fully buffing one person can cost lots of MP.

That's it, you don't even need to refine anything.

Jewelry - sockets with Topaz. Anything that adds INT is your goal and since Topaz adds INT...

3. Skills.

Full Support or not is main question here. Still, you want everything from Meditation tree, except for Curse and Bon(e)fire. For a Full Support Muse, no other skills are needed, maybe level or two in Spell Mastery for MP decrease, and possibly lvl 1 mana bolt for questing purposes(which cost just as much as Curse, but is far superior). Another interesting thing would be Fire ring(just lvl1), to indirectly increase damage done by your party members - however, this mostly helps with big monsters, not some weaklings that die in 3 hits anyway.
If you want to have make more PvP oriented Cleric, you should get both Staff Stun and Lightning. It will take a few skillpoints, but double stun is very good in 1vs1 - you will not have powerful spells like Magicians, especially Silence, but with your buffs you can still do some damage. Mild would also help a bit - damage decrease is surprisignly high, but that's 2 more levels into Spell Mastery, so quite a few SPs. Also, at least 1 lvl of Mana and Icebolt - when boosted by Damage Support they can do reasonable damage and 1 level is fairly cheap. Summons are good in theory, but Phantoms can't hit anything unless buffed and Elemental takes too many SP to be worth of it.


Muse skills:

Meditation - your BASIC skill. No big brainer here, you *must* have it at 10 before level 70. More mana, more mana regen, needed for every other skill.

Cure - heals one target and cools down pretty fast. There is no set level you should have in this skill, just level it when you feel that current level is not enough. I had lvl5 at 109 still and it was enough(you simply start healing earlier )

Healing - heals less then Cure, is more expensive, cooldown is longer, BUT heals every party member in range. Plus you can combo Cure-Heal-Cure if needed to quickly heal one person. Again, level when needed.

Lesser Haste - faster running, needed for all offensive buffs. Level 4 is enough, since it lets you get all of them, and further MSPD increase is minimal.

Power Support - THE skill for Support Muse. Higher ATK is what everyone wants, and is one of two main reason for partying with Support Muse in the first place(healing being other one). Get it to 5 as soon as possible. It also works with spells, so make sure you buff Attack Muses as well.

Hit Support - somewhat less important than previous skill, it is still quite helpful, especially if you plan to fight orange/red/purple monsters or maybe some hawkers on PVP. Lvl 3 is max for Muse, and helps a lot. It also works with spells - these *do* miss on red/purple mobs, so buff Attack Muses.

Blessing - good defensive buff, especially to Hawkers and yourself - extra HP is very useful. Level 5 is enough to get you all other defensive buffs if you want them, if not, just get one and focus on offensive skills after you become a Cleric.

Enchant Armor - it may last short, but defense bonus is huge. Even level 1 is good for some nasty situations, you need level 5 for two next buffs. Get at least level 1 if you don't want them.

Blessed Mind - quite possibly weakest buff, it gives additional mana, but you still have to use potion to regain it. It is only useful when party levels up, which isn't really often. ONLY get it if you want Evasion Aura, otherwise it is waste of SP.

Bonfire - amount of mana restored is low, and good party rarely has time to stay in one place anyway. If you *really* want it, get it to some higher level, where it would actually matter, otherwise, it is waste of SP.

Curse - this spell is probably worst attack spell in game. Low damage, low chance of poisoning target, low poison duration. DON'T GET IT. Just get mana bolt instead if you really want to.

So, to sum up, by level 70 you should have:

Meditation - 10
Cure - 5
Healing - 5
Lesser Haste - 4
Power Support - 5
Hit Support - 3
Blessing - 5 - look below
Enchant Armor - 4 - only if you want to haveDefense/Magical Aura
Blessed Mind - only if you *really* want Evasion Aura - 2
Anything else is optional.

Cleric skills:

Battle Support - get one level right after 70 and leave it there. Leveling this skill is best left for later, when your INT will be high enough to give +4ASPD per level, which is actually notable. Still, at level one it give around 30, which is very good.

Damage Support - THE skill for Cleric. Level 1 adds 14% damage to every attack and offensive skill, which isn't seen in ATK increase, but is added when damage is inflicted. As for maxing it, each level adds 2%(and 10 - 3%), so you should think if it is really worth of it. It's effect increase anyway, since character increase their ATK every level(at least they should ). Needs 19 Meditation for lvl 10 Power Support.

Critical Support - not as good as Damage Support, since skills don't critical, and by the time you get it, people will probably use them *very* often. It is mostly helpful with fast attacking character(raiders) or those without many combat skills(Artisans). Also, it helps with summons, since those use mostly physical attacks. Needs 20 Meditation for lvl 10 Hitting Support.

Defense Aura - even level one is good, since +40DEF for entire party for 30SP is quite good trade. Unfortunately, duration is shorter than other buffs(which is problem with all 3 Auras)

Magical Aura - less useful than previous one, but helps with some stronger monster, like Tyrants/Mammoths/Behemoths/Astarots. It is also said that it improves chances of resisting debuffs from ghosts/other players. 1 level is enough, since normal DEF *also* reduces damage from spells(even though it sounds stupid)

Evasion Aura - that skill would be good if it didn't have such horrible prerequisite. AVO bonus it gives is not really enough to let low DEX character avoid much, it mostly helps them against blue/green monsters. For hawkers, it could help more.

Ressurection - ideally, no one should die, but, as we all know, "sh7t" or, in most cases, triple cast from mammoths, happens. If you want to get it, get at least level 2, since it doesn't recover any EXP on level 1. Also, DO NOT CAST IT NEAR AOE USING MOBS. Ask other party members to draw it away or do it yourself with Blessing Armor. There is nothing more annoying then getting ressurected and getting killed again by AOE before you could heal.

Purify - it might seem a waste at first, since there aren't many debuffs worth dispelling, but you will appreciate it when your main tanker gets surrounded by enemies he could easily tank normally, and then gets -DEF and -AVO(tested *many* times against Goblin Guardian as well as Astarots;) ) Helps against Tirwins/Elect Ghost as well, even -DEF from BK. 1 level won't hurt(much). Also, certain monsters on Eldeon use Silence - so if your main fighter gets Silenced, you know what to do.