If you already have a wheel made for single-speed or fixed-gear riding, it'll have a spacing of 120 millimeters. Older frames with horizontal dropouts usually have a 126-millimeter spacing so there's going to be six millimeters you're going to have to make up for (which you do with spacers between the cog and locknut).
I have a common flip-flop hub (both coasting and non-coasting cogs) so I tried a spacer on each side at first. Unfortunately, when I tested the wheel in the frame, the wheel was too far left, so I put both spacers on the coasting side and things lined up. You might have to experiment, and it's important to remember to put the spacers right up next to the cog, between the two nuts that touch the frame.
If nothing works or you don't have a long enough axle, you'll have to replace it or redish your wheel. Both are double-black diamond moves best left for a seasoned mechanic.
You could also re-use your old wheel with all the gears, but it's pretty complicated and you can find cheaper wheels pretty easily. If you want to know more about that, check out Sheldon Brown's essential notes.