Ah, no worries, we all have to learn this. It's the most important safety rule: eye protection on before you go into the game zone; and mags out, guns cleared[*], and selectors on safe before you enter the safe zone.
If you're bringing your own guns rather than renting, then I'd listen out for a shout for rentals on your first day. Tail onto that, and listen in to the rules-and-safety briefing that they're given. If in any doubt on the day, just ask any marshal or other player - airsofters tend to be a friendly, helpful lot, if somewhat prone to magical thinking and wild mis-estimation of ranges.
[*] anything that doesn't need a mag to fire, shoot it at the ground until you're sure there's nothing left in the chamber.