Count the number of rear cogs on your bike. That corresponds to the width of the chain that you need. It might say something like 6-7-8 speed chain.
When you buy your new chain it will be too long. You'll need a chain tool to cut it down to the right length. Assuming your old chain was the correct length, the easiest thing to do is to compare it with your new chain to figure out where to cut. Your new chain will actually wind up being a tad shorter than the old one, but you'll figure that out.