It depends on the type of wood and what type of paint was on it before.
If the previous paint is oil based, you will need to either seal the surface with a primer/sealer (Kilz is good for this) if you want to use something other than oil based.
The oils in oil based paints soak into the wood making it very difficult to use non-oil based paints.
You can test the surface after sanding it. Just take a wet rag and drop a bit of water on the wood. If the water soaks into the wood, then you can use any type of paint you like. If the water drop bubbles on the surface, you will need to use an oil based paint.
Take note, make sure you are not dropping the water on a knothole as they tend to contain saps which will bubble the water.
Prepping the surface is the most work but it is what will yeild the best look. What type of look are you aiming for? High gloss, satin, or flat?
Don't just try and sand the old paint off. Use a chemical stripper to remove the bulk of the paint and clean the surface really well afterwards.
If the wood is very old, take a wet sponge and dampen the wood a bit. This will raise the grain of the wood so it can be properly sanded. If you do not do this, then the paint will raise the wood grain up and cause your finish to not be flat. You may have to repeat this process depending on what type of wood it is.
Sand the surface with a 400 grit dry sandpaper to prepare for primer. Alway prime the surface, never paint the wood directly, this will yeild the best results and a more durable finish.
After sanding, prime the surface with a quality primer that can be sanded. Let it dry the requisite time, and sand it using 400 grit (wet or dry).
You are now ready to paint. By the way, avoid using can spray paints. They suck. From this point it all depends on the type of finish you want after it is all done.
High gloss finishes require more work than flat or satin finishes.