i bought a new bike and i dont know how to determine the size of the chain on it. i know there are two sizes 1/8 and 3/32 but i dont know how to measure or where to measure to determine which it is. can someone help?

Look on the cog and chainring. They should say how big they are. If you have 1/32" wiggle room on a 3/32" cog, then voila! A 1/8" chain!

Measure the gap between any of the inner links. An english fractional tape measure is useful to do this.

Ask the shop from which you bought the said bike. Or look up the model online. Most bike companies have all their stats up on their websites.

Is it a "track" bike? If it were, you probably have a 1/8" size. I think that's the standard for track bikes/chainrings eh?

Though some off-the-rack, 'tarck' bikes run 3/32 drive trains. e.g Steamroller being just one.

Not true, many entry level track bikes run 3/32. For example, Bianchi Pista and Felt TK3.

Are you replacing the chain? When in doubt, get 1/8" -- it works with either width cogs or chainrings.

You can run a 3/32 chainring up front, but don't you have to have a 1/8 cog in the rear for a 1/8 chain?

Nope. 1/8 will work fine on either size drivetrain