The main healer is the one in charge of keeping the other 4 in your immediate party alive. You  do this with Greater heal, Flash heal, Renew and Power Word: Shield
There are a few rules we would like to give Priests.
First of all, if there is a magic debuff on any member, dispell it, in that dispell is a cheap instant cast spell that can get rid of 1 to 2 magic debuffs or buffs.
Secondly, it is not necessary for you to shield the Warrior except in emergency situations. Before the battle, shielding the Warrior can be very useful, it will generate no aggro and you could regen the mana before you have to heal again.
Thirdly, it is generally not a good idea to make use of the Psychic Scream in instances unless you have the area around the fight clear of mobs.
When in a group, your main priority should be to keep the main tank alive, If the main tank dies, more often than not, that's to say,  your party will wipe, this is so bad.
Make sure that you let the Main tank (MT) gather some threat before you start healing anyone. Mobs don't like people healing the people that they are trying to kill, just make sure that they are pretty pissed at the main tank.
Always, it is better for you to keep renew on your main tank. As it gives you almost no threat and heals the tank about the same as a flash heal, the battle is going generally well, you can rely on Greater heal to keep the tank alive, but if and when you get more adds, you might have to switch to flash heal to keep the tank alive.
If the monster decides that you are next on the feed to self list, then you should just fade, if that doesn't get rid of aggro then add a PW:S to your self, we don't recommend healing your self unless necessary, Run towards your MT, your warrior will gallantly save you from your measly one mob. and you have to still keep him alive with his 3 mobs that are still trying to eat him.
If the monster decides to attack someone else then you should slap a renew and a flash heal on them(and a PW:S if they are squishy), the main tank should do the rest.
But the issue facing us now is just as dire: a Healer Shortage. Above all, lots of people are still working their way up to level 80 with painstaking effort, and the majority of healers may still be leveling through Northrend. And second, they've beefed up and changed a lot of DPS builds lately, so many pre-Wrath healers may have respecced DPS to try it or to level a bit, and haven't gone back yet.
We're more of a DPS person, but we've played healers in the past. We would like to do everything about PvP healing in this game except we can't /assist on an enemy to quickly see who's being focused so we can heal them. Likewise, we have played healers in PvP many times so far and we have not had too much issue with the healing. You have to pay attention to things you are not used to paying attention.
However, you should make use of the dual-targeting system. That owns for healing. You can have your DPS focus and your heal focus. Good luck!