WP is correct. Most MOCs will require their nomination applications usually no later than October of your senior year. All are different so check each website for the exact date.
Since you seem to be in the beginning of the process, I will throw out some additional unsolicited advice. In conjunction with the MOC nomination is the Academy application. You should go online and submit a pre-application by January of your junior year. This will get you in the system and make you available for the Academy summer programs which are highly beneficial.