Go to
www.asasoftball.com and visit umpires page. Download the pdf formatted ASA Test.
Take the test with your rule book
Attend every clinic you can find
Attend a school, if available
Attend every clinic you can find
Read the Rules Supplements in the book
Attend every clinic you can find
Read the Umpire Manual in the book
Attend every clinic you can find
Observe experienced umpires
Attend every clinic you can find
Remember not to take everything literally. Apply the rules when applicable; don't try to out think the book; don't try to justify what you do or do not find in the rules or on the field. Remember 10.3.C is your authorization to rule on situations NOT covered by rule, not to misapply a rule with which you may not agree.
If you find something that seems askew, check with your local rules interpreter/uic/whatever to check out the proper interpretation. When you see another umpire perform a mechanic that is unfamiliar, check it out in the manual and then question your local uic. Understand that being a veteran official does not make everything done on the field the right thing and longevity just means someone has been doing this for a long time. Consistency really only counts when you are consistently right
Did I happen to mention you should attend every clinic you can find?
One last thing, softball is not baseball for girls or beer-bellied men. It is a sport of its own which happens to share a source of origin with baseball.