Playing Airsoft will most likely start as a hobby and if you are interested in military simulations, or just like running around a field and shooting people with fake weapons, you might decide to get some of your own equipment. The gun will come first as well as some additional batteries or gas canisters and plenty of ammo too. For practice, you might then buy a target to practice on at home or in your room.
When you get to the stage that you are wanting to use your new gun as part of a full-scale Airsoft game, you will have to buy your own armor and protection too. When playing in an organised game, you do not have the option to not wear armor as it is usually a requirement for playing. A face ask and goggles are a minimum requirement and what else you wear will be up to you. You might also want to consider if you are trying to emulate a certain type of attacking force. What protection do they wear? If you are into making Airsoft into a proper simulation, then this can be important.
Safety Glasses are a must for anyone playing Airsoft. Even if you are not thinking of playing the game outside and are just firing practice shots in your room, you should wear protective glasses. BBs from an Airsoft gun travel at very high speeds - even after bouncing off a wall at 10 meters - so you need to be protected at all times. A minimum requirement for playing a proper airsoft game is to wear eye protection. A modern automatic airsoft rifle will send hundreds of BB pellets in your direction at speeds of over 300 feet per second. If one hits you in the eye, it will most likely blind you or cause serious damage. Wearing goggles and a full face mask gives you protection from this sort of injury.
Body Armor can also be worn to help protect yourself when playing Airsoft. The small plastic BB rounds fired by an Airsoft gun can hurt if they hit unprotected skin. Even though the are much smaller than a paintball pellet, they can still sting and leave bruises on skin. Also, the bb guns gun shots are not hollow like paint balls, so they do not break when they hit their target. This means that the impact can be greater and cause more (temporary) pain. To overcome this risk, you can wear a vest that protects your body. It contains thin and lightweight plastic plates that will stop the BB's from causing too much pain. These vests also look great and can add to the overall look of a player and the atmosphere of the whole game.
One you get into playing Airsoft a little more, you will find that you have to carry around lots of things. Gas canisters, spare ammo and batteries are common for most players. These can also be quite easy to drop whilst playing and might not be easy to find afterward. Most players buy a tactical vest to help them with this problem. A tactical vest allows you to store lots of items in small pockets and you wear it like a jacket, so your movement is not restricted, like it would be if you had a back pack on. The vest does not provide a great deal of protection to the wearer, but is perfect for lugging around your spare BB's.