Somewhere north of 80% of baseball is played with fewer than 20% of the rules. If my numbers are off on this point, they're not off by much. So the average person stepping onto the field for the first time (likely to umpire his or her seven-year-old's Little League game) already brings a lot of baseball knowledge to the field. There's comfort in that ... but there's peril, too.
The point is, while you're probably pretty solid with safe/out, fair/foul, and ball/strike, umpiring anything older than about nine-year-olds is going to take a lot more experience with that not-often-called, other 80% of the rules.
And here's a really good place to start …