It's easy! Talk to your 4-H leader or extension agent, FFA advisor, or other mentor within agriculture to see how you can be judging at contests in the near future! To get an idea of what a particular judging event is like, look around the site or Google whichever event seems interesting, or for a firsthand account, talk to a fellow club or chapter member or showman to hear their stories!
If you are not in 4-H, and are interested in joining, Google "4-H in (your county name)" or visit this link on florida4h.org for a link, email, or phone number to your local 4-H office. 4-H is open to all youth ages 5-18 as of September 1st of the current school year and is in every county in Florida.
For FFA, talk to the agriculture instructor(s) at your middle or high school, or them or your guidance counselor for information on becoming involved in the agriculture program at your school. To find an FFA chapter near you, visit this link on FFA.org for information. Alternatively, Google "(your school name) FFA chapter" to look up contact info. FFA is only open to students in grades 6-12 regardless of age who are enrolled in agriculture related classes, and not every secondary school in Florida offers agricultural education.