Use a thin flat-head screwdriver or a pocketknife to dislodge the drain plug from the sink, if you use a rubber stopper. This will only work if the drain plug is not connected to the sink. Place the screwdriver or pocket knife into the edge of the plug to pry it up. You should be able to slide the knife between the drain plug and sink drain.
Look underneath your sink, if your stopper is connected to the sink. Look for a vertical metal rod. Grab the rod and gently shake it. You should see the stopper move slightly.
Pull the metal rod down. The stopper should pop out of the drain.