Assuming you are talking about a pop up sink plug that is common in many bathroom sinks, the following instructions should help. Keep in mind that there may be some variations depending on the model of your sink, but the general idea will be the same.
1) Locate the knob or lever that controls the pop up plug. This is usually located near the back of the sink near where the drain pipe meets the sink.
2) Gently pull up on the knob or lever to loosen it. You may need to use a little bit of force, but be careful not to break it.
3) Once the knob or lever is loosened, you should be able to lift up on the pop up plug and remove it from the drain hole.
4) If there is any debris or buildup around the drain hole, you can use a brush or other tool to clean it out before replacing the pop up plug.