The gun itself is obviously the most important tool in airsoft. It can make or break your performance and enjoyment. If you are serious about airsoft then make sure that you get a good gun. Lower priced guns can be fun for a while but most likely they will not last for more than a couple of months and before you know it you will have to buy a new gun if you want to keep playing. Having a quailty gun will also make you more competitive. A low quality gun which frequently jams and does not shoot straight is not much fun in the heat of battle. You will spend more time messing with the gun than actually acomplishing your objectives
There are three main categories of airsoft guns: spring, electric, and gas.
Spring guns are often the least expensive, and in some cases the most reliable. A quality spring airsof gun will practically last forever if you don't abuse it. This is becaus there are few moving parts within the gun and there is not much which can go wrong. The drawback of spring guns is that they need to be manually cocked before each shot is fired. This makes them good starter guns, but if playing against someone with an automatic AEG then you will have a big disavantage because they will be able to pump out bbs 100 times faster than you. In serious airsoft battles the only time you really see spring guns used is for sniping. Snipers do not rely on a high rate of fire but instead use tactics, cover, concealment, and marksmanship to take down their prey.
Gas powred gun use either green gas or CO2 as propusion. In other words, the gas is already presurized. This makes them easy to use and very realistic. They can fire in semi auto mode, and sometimes even in full auto. The drawback is that once you're out of gas then the gun is basically useless until you buy more. CO2 is easy to find but Green Gas usually has to purchased online or at a specialty store. There are special adapters which allow propane to be used instead of Green Gas.
The third type of airsoft gun is electric. For rifles this is the most popular type of gun. They are battery powered. The battery turns an electric motor which in turn powers a pistol which compresses air, which is used to propell the BBs. This makes them very powerful because they can shoot in full auto mode for as long as the trigger is pulled back. Many AEGs also come with a high capacity magazine which can hold 300 rounds or more. The batteries are almost always rechargeable so once they are out all you have to do is put them on a charger for a few hours and then you're ready to play again. In matches and events which last for a day or more an is a good idea to have a spare battery or two to use while the other is charging.