Detach the cabinet doors from the cabinets. This is as easy as taking the screws out that hold the doors to the cabinets.
Use 120 grit sandpaper and sand off any finish or stain that might remain on the wood. You might have to use a medium grit (they are sold as medium grit) sanding sponge to smooth the grooves of any decorative work.
Complete the sanding with the 120 grit sandpaper and then sand with a much finer grit, such as a 220. Orbital sanders with 220 grit pads work well for sanding large and flat suraces. Take your time with this part because a poor sanding job will show up after you stain the wood.
Paint the cabinets with a primer and then sand again with a 220 grit paper to prepare the wood for painting. If you are going to use a stain or a varnish, you will not want to apply a primer. Simply apply the stain or varnish after you are done with your final sanding.