I can't tell by the way the question is worded if you want to apply poly or cover over it.
Assuming you want to paint over furniture that has been coated with poly: You can sand it lightly and then prime or get adventurous and skip the sanding (like I do) and use a good adhesion primer right over the shiny surface. I use Sherwin Williams Adhesion Primer. (Or Bonding Primer...they keep changing the name.)
Good advice Sandra.
Deborah, a safe step to take is to use deglosser. This will clean and dull the surface. Water base deglossers are now available.
Then you are ready to use a primer and two coats of finish paint.
use annie sloan chalk paint, or homemade versions and you don't have to prime or sand.