First, clean the door surface with sponge and detergent; then rinse. (For the best appearance, if you can, first remove the door knobs and any other hardware; otherwise, apply masking tape to the hardware to keep primer and paint off it. Also, priming and painting will go easier and appearance of the job will be better if you can take the doors off their hinges and place them flat on a table or saw horses for the priming and painting. Be sure each surface is thoroughly dry before turning over to do the other side.) Often these type doors have a somewhat rough surface, because of the nature of the factory finish that had been applied. If yours are this way, then sand them lightly using fine (#220 grit) garnet sandpaper; wipe off with a damp rag. Then apply a latex all-purpose stain-blocking primer using a high quality synthetic nap roller.
Apply the primer evenly and in a fairly heavy coat. Use a drop cloth on the floor. Let the primer dry over night. Then, using the same type roller with a 3/8" nap, apply a top of the line interior latex paint in an eggshell, satin or semigloss finish, depending on the appearance desired. If the uniformity of appearance is not acceptable, apply a second coat of paint after the first has dried 4-6 hours. Remove any masking tape you've applied, within two hours of applying the paint. If possible, allow the paint to dry over night before closing the doors.