if it is just stain you shouldnt need to sand it. If it has a varnish over it, you will. They sell de-glosser agent you can apply over varnish that allows paint to adhere to it, but i have yet to try it. You just need to get rid of the gloss. If you choose to sand i would hit it with 80 grit paper. If you want it smoother go up to 100, but 80 is a good starting point.
It does have varnish. Couldn't think of what to call it in my first post. Almost like a clear coat. I'll look for it and see what I think. I don't mind using a little elbow grease, so atleast I have another option besides sandpaper.