You say the doors are dark brown. Are they 'stained' brown or are they painted brown?
W/O seeing them, you want to sand them anyway. If they're glossed-over with polyurethane, they should be sanded. If they're painted over with a glossey paint (not a flat paint), they should be sanded. Then, prime them and paint them.
Look in the Yellow Pages under PAINT - RETAIL for a paint store near you. Drag one of the doors in w/you and show the Retailer what you have. They'll guide you from there. Don't be afraid to put on 2 coats of Primer/Sealer! In fact, I recommend it. If you don't, you may see the brown 'bleed through' the white you eventually use as a Topcoat. And plan on 2 coats of topcoat too.