Originally Posted by
Scrodzilla
The OP isn't looking to install a fixed cog.
As Kayce pointed out, if your wheel has a multi-speed freewheel ( I think it does) - instead of a cassette with individual cogs - all you may need to do is remove it and thread on a single speed freewheel. Either way, go to a bike shop.
+1 on the bike shop. And I know the OP doesn't want to ride fixed, just putting in my two cents (as usual).