In most cases, there are three numbers at the bottom of a check, and your account number is the one in the middle. However, occasionally you'll find checks that use a different format, so it's a good idea to confirm your account number before trying to use anything that you grab off of a check.
A good way to find your account number is to look for this symbol: ⑈. Your account number appears just before that symbol. For more information, see Where is the Account Number on a Check?