I have a lite at the top of the stairs with one switch for it at the top. I want to and a secong switch at the bottom of the stairs. I believe the incoming power it at the switch (top of stairs) How do I add a second swich to control lite?

What you want is the classic 3 way switch. You need two, 2 pole switches and a 3 wire cable either between them or to the light. There are a number of ways the switches and light can be wired up. I am not going to try to describe them. Any basic wiring or general DIY book has diagrams. I am sure they are on the web, but can't give a URL.

Study the diagrams and figure out how to wire it up the way calling for the easiest way to run the cables. The same books and web sites should show some tips on fishing wires through walls. Wire Mold and other brands sell conduit and boxes meant to mount on the outside of the wall. They are fairly expensive and give a tacked on look, but are safe, easy to install, and meet code. All connections must be in a UL listed box.