I am planning on leveling a priest and i want to heal in dungeons but I don't know anything about priests. Which talent tree should I choose and which abilities should I use.
Priests have two healing trees, Discipline and Holy. Disc is mostly single target heals and strong shielding spells while holy is about being able to do everything very well, switching specialities mid fight thanks to Chakra.
If you are going to spend all your time in instances then pick the one with the style that you prefer, although the full versatility of Holy will not be available until you get into the upper levels. If you are doing both instances and some solo grind, make sure you get duel spec at L30 so you can switch between heal and damage specs.
Some good, basic priest healer introductions can be found on WoWInsider.