Starting a priest w buddies, what talent tree?

Hello guys,
This seems like the place to ask the question.
Me and 2 of my friends will be starting to play today, they are completely new to the game, while i have decent experience of the game. Tho i have played just a mage. I wanna start a undead priest, and i've been doing some google-ing but i got completely confused.
What talent tree is best? I wanna heal in RDF but also do some kinda dps. Ive heard that holy is only for healing, shadow is only dps and discipline is... I dont know. So i could use some help about what talent tree is about what. (We're playing on Lordaeron)
Thanks!

Get your first 3 points in Spirit Tap. Then you can go full Discipline. That's what I'd advise, as in the long term it is the most mana efficient. As for doing damage, I'd advise using your wand unless you just have loads of mana to use.

I leveled my priest with full shadow and I healed RDF up to lvl 60 with no problem, so if you don't want to level as a healer know that you'll be fine healing in dungeons in Shadow Spec.

Also, as Mercy said, Spirit Tap is a must for all specs when you're leveling.