In WoW what is the best talent tree and stats for a priest healer

Priests have two healing trees, Discipline and Holy. Which one of these is best would depend on your preferred playstyle.

Discipline
Discipline priests are masters of Power Word:Shield, and gain bonuses when using their shields, such as a bonus to haste or gaining mana when their shields are destroyed. As tank healers, Discipline Priests are wanted because of their big, steady heals, as well as a damage reduction buff on targets they heal.

Discipline priests prefer Intellect and Spellpower as stats above all, then some Crit and Haste. While they gain additional mana returns with a higher Spirit, they need less of it than Holy Priests. With their immense Intellect scores, their mana pools usually outlast the fight.

Holy
Holy Priests are very versatile, and have an immense toolkit including AoE healing, Lightwell and Guardian Spirit. They are a bit more difficult to learn to play than Discipline, but reward this with a large healing output and the ability to switch between raid healing and tank healing without a hitch.

Holy Priests prefer Spellpower, Intellect and Spirit, with an equal dose of Haste and Crit. Critical heals give them special bonuses with their talents, and with Haste their heals tick faster and cast faster.

Again, which of these you prefer depends on whether you want to mitigate damage or have a very versatile healer type. Once you have determine what you'd like to play, I'd advise looking up a few blogs for that type of healer to learn about healing spells, tips and tricks and how to gem/enchant your gear.