you can use the fir/pine plywood.go to your local sherwin williams paint dealer and get some "Tile clad II" 2 part epoxy paint...i like to use black;but white is ok too.just make sure that you give it 2 coats about 24-48 hours apart.
I would paint it with either an epoxy paint as loha suggested or a waterproof/mold resistant paint. Also, I wouldn't paint it white, as it will get dirty and look bad.
__________________ 55 Gallon Tank (Display):
-18 Community Fish (Angelfish, Blue Gouramis, Gold Gouramis, Peppered Cories, Pleco)
40 Gallon Tank:
-4 Pseudotropheus Cichlids
20 Gallon Long Tank (Display, Planted):
-13 Community Fish (Zebra Danios, Neon Tetras, Ghost Catfish, Betta)
-2 African Dwarf Frogs
-1 Black Mystery Snail
10 Gallon Tank
-1 Pseudotropheus Cichlid (permanently injured)
10 Gallon Tank (Fish Only Saltwater)
-2 Percula Clown Fish
10 Gallon Tank (Planted):
-Lots of Crystal Red Shrimp