If a sink or tub is sluggish, run hot water through the drain. Check the strainer (if there is one) and clean out any hair or debris that may have collected. If water still drains slowly, the next step is plunging.
Plunging works in two ways: by pushing a clog through to the stack, and by pulling debris back up into the sink.
For tough jobs you may want to try a pressure-type plunger, which looks a bit like an accordion. It generates greater pressure than a standard plunger.
Before plunging make sure the water has only one exit point -- through the drain. Plug overflow holes and clamp connecting hoses before you begin.
Sometimes plunging works easily, with little mess. Other times water sprays all over the place. Be prepared to wipe up substantial amounts of water.
If the water suddenly drains out, the clog has passed out of the drainpipe and into the stack, or the plunger has sucked debris back into the sink. Pull debris out of the drain and/or sink. If the drain remains clogged after numerous attempts at plunging, move on to dismantling the trap.