What To Wear
You can wear whatever clothing you want to in most games, but one thing you ALWAYS need wear safety glasses/goggles. Most pro airsoft games require you to wear a full face mask, but anywhere else, glasses are OK. Since the glasses only cover your eyes and getting shot in the bare cheek stings, you might also want to wear a bandanna over your mouth.
To protect your head from head shots, you may want to wear a hat. If you are playing in the sun, wear a boonie hat. For colder weather, wear a beanie. If you want that military feel, wear a tactical helmet.
I've seen people airsoft in just a light T-shirt, and they don't seem to mind. However, if you don't want a million tiny welts of your back, I suggest wearing something a little heavier. Most people are fine in a long sleeve shirt, but if you have a low tolerance of pain, wear a sweater or jacket over it.
I personally think that a hit on the calf or shin hurts the most. To protect your legs, wear long pants that fit loosely around your legs (no skinny jeans). This is because the loose material will absorb a lot of the BBs energy before it hits you in the leg. Skinny jeans do not offer that, it's like getting shot in that bare leg. Your best bets are boot-cut jeans, exercise pants, or cargo pants.
Sneakers are fine for airsoft, but especially in the woods, you'd be smarter to wear boots. This avoids injury to your foot, like a twisted ankle, because boots are tougher than sneakers. If boots sound a little too heavy and you feel more comfortable in sneakers, wear an old pair. Unless you're willing to get your new ones dirty.
Most beginners start out with a backpack to keep their stuff in. You can keep extra ammo, a secondary weapon, water, snacks, whatever they'll need for an airsoft war. As they become more committed to the sport, they might end up getting a tactical vest to have what they need in easy reach. Real tactical vests offer a bit of protection from BBs, but they can be fairly expensive. For a cheaper alternative, get a fishing vest. Although this offers very little protection, it's light and has pockets to store what you need to take with you.
You can get a bunch of accessories for airsoft, but beginners only need a few. You may want to invest of some bike gloves because knuckle shot hurt. A lot. The ones without tips work best because they allow you to pick BBs up one at a time for reloading. If you do have a secondary weapon, you might want a holster to have your gun in easy reach. Knee pads are very helpful, especially in rocky terrain because they protect your knees when you need to kneel and shoot.
Now that you know about the different things you can wear, you'll need to choose what you will wear. Here's an example of what the average beginner should wear: safety glasses, sweatshirt, loose jeans, old sneakers, backpack, and gloves. However, in the end, it is up to you to choose what feels most comfortable.