How to Clear a Sink or Tub Drain

A slow-draining tub or sink usually means the drain is clogged. On a sink, first try using a plunger, and, if that doesn't work, clean out the trap

While you're at it, you may discover it's time to replace the entire trap. You may save a little by buying only one piece, but you'll pay for it later when you have to repeat the labor. When reassembling the trap, be careful not to over-tighten the slip-joint nuts.
A tub is often fouled with hair. Pull out the pop-up stopper (if it has one) by wiggling the lever and lifting the stopper. Ease it out, and remove hair from the linkage.

Otherwise, use a blow bag or snake out the drain.

If you've done both, be sure the vent line isn't blocked with a bird's nest or other debris. You can snake it from the roof (but be careful not to drop the snake into the pipe!).

If you're on a septic system, consider that your tank and/or drain field may possibly be overloaded.