3 Pack Drain Snake Clog Remover Drain Snake, FrontTech Drain Snake Hair Drain Clog Tool, 19.6 Inch Drain Snake Drain Relief Cleaner Tool for Sink, Tube Drain Cleaning

Crud decomposes in the drain. If you've never cleaned out the drain, start there. Get one of those plastic sink drain/hair snake things (like this) and use it to remove all the gunk. You might want to wear gloves and have a plastic shopping bag to put said gunk in. Then run hot water down it to rinse. This operation takes maybe 10 minutes, not counting the time you spend acquiring the tool. It might be sufficient, but if it continues to reek, you can unscrew the p-trap and clean that more thoroughly. Do not put Liquid Plumr in there, it could cause more problems than it solves. It is not safe for all pipes. Especially if you are renting...if you do the above steps and it's not helping, contact your landlord or building manager.