Try taking the P trap off that's under your sink. The P Trap should unscrew with a pipe wrench or large pair of pliers.
Most times that P trap that keeps sewer gases from coming in gets clogged with hair or other things.
If that doesn't fix it then your air vent to your drain line may be clogged with something as well or the rest of the drain itself might be clogged.
You can get a plumbers snake at the home depot or lowes and run it through the pipe to clear any clogs that might be in the drain.
You could also get some drain cleaner and see if that clears the clog.