I would suggest looking at
http://www.naso.org/BeOfficial/.
It talks about getting certified by your local organization. Being a good umpire is knowing the rules, but also knowing how to umpire. That includes mechanics, feel for the game, and understanding the strike zone.
Most of the above you can only get by working games and learning from your experiences and from other umpires.
After 10 years, I am still learning from more experienced umpires.