Go to your local pro paint store and ask for there wood primer(I prefer oil based), if you are going to brush it tell them. Get a good paint for the job (I use oil based for doors for durability, but you don't have to) Use wood filler if nesasary for nicks and dings, sand before primering. Prime the door one brush lenth at a time all the way to the bottom(to help eliminate lap marks) work quikly. After this dries sand lightly with 220 grit paper, dust off with a tack cloth, and paint 2 coats(allow each coat to dry completely, if oil based I recomend waiting till next day to recoat). Start with the edges then the face of the doors. Sand lightly between coats.