First you need to know what kind of role your gonna play whether its sniper, rusher, camper, or support gunner.
From there you will need to choose a price range. Next you will need to choose the type of gun you want.
Cheap guns are around $100
A decent gun is $150-200
Good Guns are $200+
Cheap guns would be good for young children 8-10.
A decent gun would be good for a beginner or a experienced player that plans on upgrading and modifying
Good guns are recommended for experienced players. When you have an expensive gun you should clean it often.
Before buying a gun you need to check if it comes with a battery and what kind of battery. If the gun comes with a battery and you are a beginner to airsoft you should probable stick with the battery it comes with usually a 8.4v. If the gun comes with no battery you should check if the gun is lipo ready and if so consider a lipo battery but first read more about it on a different blog or website. If the gun is not lipo ready then you should get a 9.6v for an average rate of fire.
For a cheap gun just go to Walmart and see what they have there.
Decent gun brands are G&G, JG, Matrix, and Echo 1 USA.
Good gun brands are Ares, KWA, Tokyo Marui, G&G, H&K, Classic Army, and ICS
Type of gun is all up to you. The only thing I can tell you that you should take in mind is that m4s are the most upgradeable. Other then that the type of gun is for your liking.