go to pcpartpicker.com and select the exact pc components you want/have and it will tell you the amount of wattage those components need. It is recommended that you get a PSU that has about 100-300 watts more than you need just in case you wish to overclock or upgrade parts later on.
Brands to consider: Corsair, EVGA, Antec, and Seasonic are all pretty much guarantees of being good PSUs.