respec the priest to holy, you can't do instances without a healer unless you have passive heals (shaman totems) or massively overgear/overlevel the instance. You should have buttons to heal every member of the party with big heals, and buttons to perform group heals (CoH, POH, etc). Most of the time your priest will just be healing the tank, but you will heal targeted players occasionally. So for example if mage#2 is tombed you want to alternate between heal tank, heal mage#2, heal mage #2, heal tank, etc.

think long and hard how you want your keys to be setup. for example you may want to use 8 keys:

1 = tank initial attack sequence, does nothing on healer
2 = tank/dps spam macro, does nothing on healer
3 = another dps button, does nothing on healer
4 = whatever
shift-1 = heal tank, does nothing on tank/mages
shift-2 = heal party1
.. etc

that way you start fights with a few presses of 1 (just tank attacking) and shift-1 (healing tank), then just switch to mashing 2 and shift-1 (dps, tank, and heal tank) for the rest of the fight (pressing shift-2, shift-3, etc as mages need heals).

There are a million ways to set it up, I have about 10 keys that are exclusively used by my tank (rftgyzxc + shift-same), 12 keys used by my DPSers (2-8, shift2-8, alt2-8), and 5-6 keys used by my healer. it lets you tackle pretty much anything and adapt quickly.