A few tips:

Cure 1: Your main healing spell, always. This should be your go-to spell for normal damage being thrown around the party.

Regen: Your 2nd main healing spell. Should always be kept up on the tank, and occasionally tossed onto DPS or yourself in fights that have random single target damage being thrown around.

Cure 2: Should be used in heavy damage situations against the tank. However keep in mind that most bosses will have spike damage followed up by something that gives you a little downtime to heal. You must gauge the right time to use this. Most of the time you can cast Cure 1 instead. Also great to use when Cure 1 procs the free buff.

Medica: Good for your first AE healing spell, not great, but will get the job done for the levels you receive it and actually need it. Keep in mind the range isn't very big.

Medica 2: Your best AE healing spell (imo). Has a bigger range then Medica and adds a Regen type effect to it as well so you don't have to worry about topping people off after some nasty AE damage.

Cure 3: Situational. The mana cost is only worth it if you actually have people stacked up. Typically not your go to spell if you happen to be running with random people. Great for specific AE fights where you know people will be stacking on top of each other (such as Garuda for example).

Benediction: Your O-**** button, saved for the tank most of the time, and sometimes yourself. Should only be used in extreme cases.

Shroud of Saints: This is your mana regen buff, and should be used accordingly. It also reduces your current enmity by half, but for the most part should only be used to regain your mana back. You must get a feel for this spell, as the amount it regenerates will vary with your gear/level/etc. Use it when you know you will not waste any of the mana it gives back. For example if you are sitting at 4000/4500 mana there's no point in using it, you will waste the buff. However if you have maybe 3000/4500 mana, now would be a good time to use it, so you can start the cooldown, and have it back up when you need it again.