There are many ways to unclog a sink. One of the first things to try is a pot of boiling water. Pouring boiling water down a drain can often unclog a sink stopped up by grease and gunk made up of hair and soap. It's best to use this method only on metal pipes, however, as PVC pipes can actually begin to soften when water reaches temperatures above 175 degrees Fahrenheit (79.4 degrees Celsius). Also, take extra care with carrying a pot or kettle from the stove to your sink to avoid burns.
If you cannot unclog a sink with the boiling water, try a cup plunger. Before you begin, stuff a wet rag into the holes on your sink's overflow outlet if you're working on a bathroom sink; they are usually located at the top of the sink bowl. Alternatively, you can use duct tape to seal this outlet. Put the plunger cup over your drain and make sure you have a good seal that is right up against your sink's surface; then push the plunger up and down with quick movements, periodically checking whether your sink is beginning to drain. In many cases, this plunger method will unclog a sink.
If you are not able to unclog a sink with a cup plunger, you can try a sink auger, which can be found at most hardware stores or home-improvement shops. Remove the sink stopper or drain basket, and feed the auger cable into the drain hole. Eventually, you should feel the clog and pull out a foot (30.4 centimeters) of additional cable, tightening the screw on the side of the auger. Now turn the crank handle and extend the cable until you feel that you've pushed the clog out. You may have to repeat this method, following up by flushing it with hot tap water once it appears that the clog is fixed.
If all else fails, you may need to remove the waste trap to unclog a sink. Placing a bucket beneath your sink's waste trap, use a pipe wrench or channel pliers to loosen and remove the slip knots located at both ends of the trap, removing the trap from the rest of the piping. Place the auger cable into the pipe until you feel the clog; then extend another 12 inches (30.48 centimeters) of cable into the pipe, tightening the screw and cranking the handle. You should extend additional cable until the clog is cleared. Once this has worked, reassemble your waste trap and use hot tap water to flush out anything that remains in the drain.