Generally speaking, if you keep yourself at 90 degree angles to tag plays, you should be enough out of the way from a throw to another base. ASA stresses "angle over distance." Can't speak for NFHS, but I think that motto applies to all ball, not just softball.
Keep your focused eye on the baserunning responsibilities of the runners, and keep the ball and defensive players in your peripheral when they're nowhere near the play or the runners.
Stay as alert as possible. Anticipate, but don't assume.
If you have multiple runners that are your responsibility, the closer the runner is to being the lead runner, the more you may want to devote a little more attention to her. After all, that's what the defense is going to do as well.
Resign yourself to the fact that you will make mistakes, but push yourself to embrace them and allow them to make you a better umpire.
But above all, look through your rule book and find the BU mechanics. Study them as best you can. Go to clinics and don't be afraid to ask questions. Find someone in your organization whose opinion you can trust (and older does not always mean "better"), and ask them questions. Umpires love to give their opinions.
And definitely come back to this forum. A LOT!
Welcome to the softball forum!