A smelly sink is almost always organic buildup inside the drain — hair, soap scum, food particles, and bacteria. This is a quick clean, not a plumbing problem.
Pull the drain stopper out — most twist counterclockwise or lift straight up. This is almost always where the smell is coming from. Scrub it thoroughly with a drain brush and hot water.
Pour a full cup of baking soda directly into the drain. Pour a cup of white vinegar in immediately after. Let it fizz for 10 minutes.
Run the hottest water for 60 seconds to flush everything through. Put the stopper back in place. The smell should be gone immediately.