Joined: Jun 2012
Posts: 1,881
Likes: 188
From: Colorado
Bikes: '91 Mtn Tek Vertical, '74 Raleigh Sports, '72 Raleigh Twenty, '84 Univega Gran Turismo, '09 Surly Karate Monkey, '92 Burley Rock-n-Roll, '86 Miyata 310, '76 Raleigh Shopper
You'd match the thickness of your cog to the thickness of your chainring and chain. I've heard you can use an 1/8" chain with a 3/32" cog, but I'd think it'd be noisy and skippy.
Chances are good, though, that you have a 1/8" chain, unless the bike is fairly new or you're using a modern crank and chainring.