Most umpires begin in the youth leagues. You may think you can jump right in and umpire pro games 'cause you are a hot-shot ball player and caught at the DI level and still are real good at slow pitch softball and can tell every time when the umpire blows a call on tv.
You'd be wrong.
Umpire mechanics take a while to learn. The basics are not very complicated, but they are hard (read impossible) to master. Most of what umpires do is reactionary and is developed over time. Do you really want to show your inexperience in front of your home town high school coach?
Start with little kids and get the experience you need. Call your local youth league and ask to speak with the Umpire In Charge, or UIC. Most league UICs will conduct umpire training before the season begins - generally February-early April. It will cover basic field and plate mechanics and may discuss basic rules. It is up to you to study on your own and learn.
Attend as many of these training sessions as you can with as many leagues as you can. Even if you end up declining games, you will have more opportunities to work and will receive a broader initial training.
Your UIC will tell you if you need to join a national or local association. If you have any ambition in continuing into higher levels, you may want to join a local umpire's association. The youth UIC can point you in the right direction.