Why doesnt my chain fit on my cog?
Chains for modern bicycles all have a 1/2" pitch - the distance from roller to roller or from pin to pin. However, multi-speed bicycles have rollers that are 3/32" wide (or narrower), while many single speed drivetrains (fixies, track bikes, BMX) have rollers that are 1/8" wide. Similarly, there are chainrings and cogs which accommodate wider or narrower chains. Since single speeds and fixies have diverged from the world of track bikes, lots of bikes are set up with parts from multi-speed bicycles (i.e. ubiquitous 3/32" chainrings, cogs, freewheels, and chains) rather than with wider 1/8" track parts.



Unfortunately, if you use a narrow chain on wider chainrings/cogs the drivetrain will bind up! The opposite is possible - a 1/8" chain on 3/32" hardware - because the chain is wide enough to fit over the teeth. It's possible that you bought a wheel fitted with a 1/8" cog to mesh with a drivetrain fitted with a 3/32" chain. The easiest thing to do would be to buy a 1/8" chain. There are different opinions on mixing component widths, but so long as it is adjusted properly it should work quietly and safely. 

Before you do anything, measure the width of your chainring teeth and compare it to the width of your rear cog teeth.