If you have horizontal dropouts, you just need to add chain tension by sliding the wheel back after shortening the chain. If you have vertical dropouts, you neet some type of chain tensioner - either a derailleur, or something like the Surly Singulator (pictured with the cassette spacers in the link below). What kind of hub are you planning to convert?

Harris Cyclery is a good place to look for what you need:
http://sheldonbrown.com/singlespeed.html#k7hub