Valve-stem leaks are almost always caused by loose valve cores. This is fixed easily enough by tightening them with a valve-core tool. This tool fits inside the opening of the valve stem. It locks onto the valve core and enables you to tighten or remove the valve core. If the valve stem continues to leak after you have tightened it, it will need to be replaced with a new one.
Remove the valve-stem cap and let all the air out of the tire.
Insert the valve-core remover into the valve stem. There is a small fork on the valve-core remover. You should feel the small fork fit over a small fitting inside the valve core.
Twist the core remover counterclockwise with your fingers. Unscrew the valve core and remove it.
Drop the new valve core into the valve stem. Insert the core-removal tool and twist clockwise. Screw the new core into the stem until tight.
Most hardware stores have valve-core removal tools and spare valve cores in their auto departments.
You cannot fix a leaky bicycle valve stem if it is a "Presta" system. These are unrepairable. The whole tube will need to be replaced.