Your statement should have your account number on it. Or your checks have your account number on em.
Just sign the back of the check you want to cash (I'm assuming it's a check someone wrote you) and put your account number under it. Then go to the bank and give them your ID and the check and badda boom, badda bing -- cash.
If you just want to get cash on one of your own checks just take your check into the bank and write it out for however much you want. Some banks like you to make it out to yourself, some to "Cash", some to the bank name. All three should work really.