I cannot give you much advice as it relates to working the umpire position.
You will be nervous, that is natural. You will make mistakes, we all did that (many still do).
You will probably communicate the most with the Referee. Listen to his explanations of penalties very carefully, he could be wrong.
Have a good pregame so that you understand who has a spot and who does not.
I am sure there are many more things, but I do not work umpire. I just know that the umpire is my best friend on the field (as a Referee).
Have fun.