Relax and have fun. You got invited to the tryout because you've showed potential. Just ref your game, work hard and don't be a know-it-all. Do what you did that got ya' the invitation.
One more thing: I do know a couple refs who got hammered for referring to players as "ladies." If you need to refer to them, using the term "players" or "number 24" will never get you in trouble.
Z