Okay, firstly this is determined by how many cogs are in the cassette. 10? Then you will need a 10-speed chain. Next you can flip the bike and put a dot of white-out or nail polish on a pin rivet. Sit on a stool and count the links as you turn the crank. If you buy the longest chain, such as a 122, no problem, just trim some off. An easy way to count is with an old chopstick. It will click with each passing pin. I will remove the old chain and hang it on a dowel in a vice, a door frame with a paper towel on top also works. Match the new length to the old length side-by-side. Remember to account for the quick-link.