I ten boxed Kara and my head still hurts when it comes to bindings.

Basically get one main button setup that is your 'open fire' button that you spam over and over and put it someplace that is comfortable to mash. I use EDSF for movement so my pinky is really comfortable resting on 'A' and that is my 'open fire' button.

[note-don't copy/paste these macros, i'm writing them from memory not copying them from my game directory]

Button 1

Tank:
/castrandom holy shield, consecration(rank1),cleanse

DPS:
/cast [target=focustarget,exists,harm,nodead,nochanneling] Spell

Once your more comfortable using macros do a /castsequence instead so you use more abilities.

Priest:
/castrandom [target=focustargettarget,exists,help,nodead,nochan neling] Fear Ward,Prayer of Mending,Dispel Magic

Button 2

Then for most splash dmg my priest (or if i'm using shamans) just use their AOE heals.
/cast Prayer of Healing

Button 3
In raids I drive with my tank but i'm always moving my mouse over to my priest's screen to heal. I use:
/cast [target=mouseover,exists,help,nodead,nochanneling] [target=focustargettarget,exists,nodead,help,nochan neling] Flash Heal

those 3 buttons should get you past 80% of the fights you'll face.

Enjoy!