How much and can I?
 

Well I picked up an old scott and want to make a SS........ I have the cranks already, so no biggie ther, but I can't afford a new rim/entire set up, so what I want to do is get a single for the rear... I just need spacers right? how much....

also what do I need for chain tension? and how much?

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

Sweet..thanks!!