Online Training and Reference Materials
If you are new to umpiring or looking for refresher training, please check out these training resources:  

Little League Umpiring 101
Visual training for beginning umpires.

Umpire Bible
The Umpire Bible is the premier website for new and developing umpires of amateur baseball, as well as for players, coaches, and fans who want to enhance their command of the rules of baseball. 

Basic Umpiring and the Two Umpire System
Professional umpire Al Kaplan guides you through the basics of umpiring.  In this 26 minute video, you'll learn how to:
1) Call Safe or Out
2) Make the Fair or Foul Call
3) Use the Two Umpire System 
4) Call Balls and Strikes from behind the plate
5) Move into position to make calls with the best line of sight