Arguably the most valuable protection gear in airsoft, the face mask will serve to protect your eyes and face from injury by airsoft BBs and other obstructions in the bush. A full face mask is recommended, however as a minimum you should have the goggles in order to protect your eyes. Losing an eye in an airsoft game would just be senseless.
Tactical Vest
For mil-sim games, the tactical vest will serve as your carry on in the field. Getting a tactical vest with many pockets for storing extra clips, BB rounds, flashlights, knives, radios and perhaps even a snack is worth its weight in gold. Also, with higher end vests, the thick material and supplies you are carry will serve to help protect you from getting hit and perhaps hurt from an enemy player.
Holsters
Depending on your intents, there are lots of holsters to choose from. For mil-sims, a leg holster might be the preferred holster, due to the player already having a tactical vest, and not able to handle an additional load on his shoulders. But for police training, a shoulder holster is ideal, providing not only a holster for the airsoft pistol, but also a compartment for additional clips, etc on the other shoulder. When combined with a belt holster, the police trainee can store that extra sidearm that may be necessary in an advanced situation training session.
Miscellaneous Gear
Some of these will come as no shock, others you might not have thought of. You should consider carry extra magazines to help you quickly reload before another attacker comes your way. Extra speed loader BB cases and gas canisters (if you are using a gas airsoft gun) will also be necessary for this quick response action. For any type of night activity, you will likely want to carry a flashlight, and perhaps laser dot scopes on your airsoft guns. Night vision goggles would also be of use in this situation. Make sure you have a comfortable pair of boots and fatigues to wear to game as well. There is no substitute for comfortable shoes, but they also need to protect your feet from rocks, sticks, bushes and other hardships out in the field. Ropes and other situation specific gear may be required.
To sum it up, if you are a beginner, focus on getting a face mask, a holster, maybe a tactical vest, good fatigues and boots will get you off to the right start. For night matches, having a flashlight and laser dot scope will be an added benefit.
