There are basically two ways to convert a bike with vertical drop outs to singlespeed. The cheapest way is to do it with a chain tensioner, a cog, and spacers. The better, but more expensive way is to get a White Industries ENO eccentric hub and freewheel. Here's a bunch of good info about ss conversion: Singlespeed Bicycle Conversions by Sheldon Brownlooking to switch my k2 mtb to a single speed. any tips/hints on crank sets, freewheels, gear ratios?
There is no other choice worth considering. A white Industries ENO eccentric hub and crank and voila you are in heaven. Reliable as a rock and a helluva lot better looking.If you decide to go with an eccentric hub, I highly recommend the matching White ENO freewheel. It is absolutely the best freewheel around. They perform flawlessly and last damn near forever.