for hygiene purposes, I recommend having your own mask or goggles.
boots with supported ankles...
depending on if you are playing milsim or casual skirmish, you may need BDUs, I played my first couple games with just regular old clothes, you can visit army surplus stores to pick up something basic, even thrift stores depending on the uniform requirements of the games.
A method to carry fluids and or snacks. Doesn't have to be a hydration carrier, a backpack is fine if you can retask something you already own. I used a MEC messenger bag for my first game.
If the game you're going to offers rentals (like mentioned above)you can decide to bring your own BBs, or purchase extras at the field if they have some.