Find which light the switch wire runs to, pull another 12/2 (or 14/2 depending on what's already there) from the existing.
How To Wire A Wall Switch. Learning how to wire a switched outlet is not difficult. How to wire a switched outlet/with close ups of wiring.
How to properly connect a ceiling fan wall switch - YouTube from i.ytimg.com
Still, you will want to replace a switch when it wears out, in part, because it presents a fire hazard. Have a look at the video above from this old house. How to read these diagrams.
As with all things electrical, safety is a real issue, so you need to know how to wire a wall switch safely, securely, and easily.
Electric wall switches last a long time. Where should you install your wall lights? The next step is to take another patch. 5 smart tips everyone should know when removing light scoring the intersection between the old switch's tabs and wall will prevent you from tearing the paint.
