I'll start with a guide to creating a new cleric, however, and lead on from there.
The meat of this guide will be the part intended for higher level clerics, though. So skip down to the next section if you're already doing dungeon parties. I'm going to illustrate how to play a *good* healer here - how to play one 'professionally'. If you follow this advice, you'll be in the 5-10% of truly competent clerics in the game.
Playing a mediocre cleric still requires a bit of attention, but playing a good one is somewhat complicated. So bear with me as I get really detailed for some of this guide. A lot of these things will come naturally to you after enough time, but it's important that you establish these good habits as soon as possible.
If you only scan one part of this guide, read the part below labeled "MP Conservation" and "Healing Strategy".
Advice on creating a new cleric
Playing a cleric at the very beginning is much like playing any other sort of character. Your goal here is just to get to level 25 or so. After that point, you should only be doing dungeon parties. You *can* solo at any level to some extent, of course, but other classes are much better at fighting. Don't frustrate yourself by attempting to do it with your cleric - just make an alt to solo with when you don't want to group with people and heal.
Skills:
Staff Mastery - (you only use a staff for the M.Atk, to boost your heals, and Staff Mastery only boosts the P.Atk mildly.. thus, it's worthless. Don't waste your JPs)
Mace Mastery - You can put 3-4 points into this to get you through rappelz if you like. After that, you'll never use a mace again (if you're a good healer). If you want to solo later on, put more points into it then.
So, do quests till you're level 25 or so. Focus on raising your heals as much as possible, along with Wizardry Mind and Mental Amplify.
As far as equipment goes...
Weapons: You can buy an r2 mace when you reach level 20 if you like, but you could just get by with your r1 mace for a few levels also. You won't be needing a mace after lvl25, so the only important thing is to have an r2 staff ready. When you go into a dungeon, don't take your mace - you should never, ever be using it. As far as using strike cubes... it's not a bad idea, but it's not a priority. It's more important *how* you heal and that you sit often, use deluxe spell potions, etc, than how much you heal for. *Don't* dump a ton of rappelz gold into getting a super staff and then fail to use deluxe spell potions. One strike cube could pay for 44 *hours* of keeping deluxe spell potions on. And higher cast speed does a *million* times more good to you than the benefit of a single strike cube. But if you have the rappelz gold, then yes, it's good to use some. Over +4 is just if you're bored or want to try making a really killer staff for your DM or something, though - you don't need a +8 or +10 staff, even if it would be 'nice'.