FFA is an integral part of agricultural education. To join FFA, you must be enrolled in an agriculture course at your school. Contact your school counselor or agriculture teacher to enroll in an agricultural education program and join FFA. Go here for more information.
Contact your state FFA association and ask about membership requirements and/or restrictions in your state.
Additional information can be found in the Official FFA Manual.
Per Article V. Membership, Section B of the Official Manual
Student Membership – To be eligible for membership in a chartered FFA chapter, a student must be enrolled in a state approved school-based agricultural education program not below the 5th grade level. To become a student member and retain membership, a student must:
A member may retain active membership until November 30, following the fourth National FFA Convention after graduation from high school.