ahhh ****, well how about that you can. on to phase two of the question then. if it is fixed/fixed, you can gear the other side however you want. if you like an easy gear but want to be able to flip it to something harder so that you can start mashing and putting the speed down, put a smaller cog on the back. if you think youre gearing now is alright, but wish you were able to flip it to something easier on those hilly rides, put a larger cog on the other side.

make sure there is enough chain to be able to tension your wheel with both sizes of cogs.

you can get cogs and lockrings from your LBS if they carry that sort of stuff. if not, click on the bike nashbar advertisement on the side of the page. they have all kinds of neat stuff.

note: if you get a 1/8" cog, you will need a 1/8" chain as well. if you are putting on a new cog, you will need to get a chain whip (or you can rotafix your paint away) and a lockring tool. or you can always have your LBS do it. where are you located anyhow?