There is really no spell order to maximize HPS like there is for DPS. You heal for the situation. If you somebody needs healing for the situation, you have to analyze and act accordingly.
General rules of thumb:
1. Keep renew up on the tank as much as possible. 100% of the time is preferred, 0% of the time should be avoided.
2. Greater heals on the tank. Tanks can and will take a lot of damage, sometimes 20k or more. If you get stuck using flash heals, you will run out of mana trying to catch up.
3. Shielding tanks is ok. Your shield may absorb upto 5,000 in damage, which negates a negligible amount of threat, when you consider tanks in heroics are likely to take 10 to 15k damage.
4. Prayer of mending should be up as often as renew. It is a passive heal but it works wonders healing who ever has the buff when they take damage.
5. DPS who pull from threat who die give agro back to the tank. Let them die, especially if they have threat reduction abilities like hunters, rogues, mages, warlocks, priests and don't use them. If they don't have threat reduction abilities and don't run back to the tank, let them die. It is easier for you to keep a tank up with one less dps then it is to keep dps up with a mob or boss hitting them for 20k and up.
Gear wise as holy, you want Spell power, spirit and haste. I've seen too many tanks die with 0.1 seconds left on the heal that would've saved them. In holy, my GHeal is down to just above 2.1 seconds from 2.5. That 0.4seconds is a lifesaver. With serendipity, I've seen my Gheal cast in 1.5 seconds.
A holy build like
this (It's my holy build, so I'm pretty biased towards it) is pretty darned good.
I love my lightwell. Even if nobody in the group uses it, I can use it to heal myself. It is better then a bandage any day of the week!
And most of all, practice, practice, practice!!!! And ask questions
eath.