How do I install a combination switch and receptacle. I presently just have a light switch? Thank You.

If all you have is a switch you probably can't add a outlet unless there is a neutral available. Do you have just the two wires coming to and from the switch?

Like ballengerb1 says. If you have only 2 wires, and the 2 wires are in conduit, and it goes to lite? Lite will have a white.Remove lite, push white wire in, or pull 1 wire out with 2 wires attached, to replace the one.
There are many combination devices including GFI's and Dimmers. However if for some reason you wanted two full size devices, Maybe an X10 type, and you have a two gang box
with a single gang mudring. Get a two gang mudring, lay over single gang as accurate as possible, draw lightly around new mudring, take razor knife and cut up to line, remove mudring screws, and install two gang mudring. It is possible to do this and with new two gang plate should cover without painting or repair to drywall.