What should I sort out before the first ball?

With the other team's umpire:

• agree who does the main role and who does square leg, and when you swap (square leg for half the match each, or main and square leg for one half each)
• agree a quick way to check with each other on close calls, such as a hand on the chest for "not out"
• remind each other of the two-fingers check at two balls remaining.

With the scorers: introduce yourself and agree how they'll acknowledge your signals (usually a wave back).

With the bowler, at the start of each new bowler's spell: ask which arm they bowl with and whether they're going over or around the wicket, so you can stand where you won't get in their way.

And check you have your hat and something to count balls with.