Are you trying to get a water mark off of a stereo cabinet? Like where someone set a glass with out a coaster. Or a water stain (dark ring) it makes a differance as one is in the finish and one is in the wood.
If it is a white ring it is in the finish, go to the hardware store and get some rottenstone/pumice, make a paste of this and water to polish out the ring...be careful not to go thru the finish. Clean and dry the area well and wax it.
If it is a dark ring your SOL. This requires more work. You need to get some oxalic acid (wood bleach) DO NOT use laundry bleach!! Using a fine brush apply it to the ring only, a little at a time and clean it up. A little more and keep repeating until the stain is gone. You may have to restain the piece and you will have to refinish it.