Step 1: Ask yourself some questions
How hardcore am I and are my friends?
If you like little pistols and mini cheap automatics, travel no farther than your local department store.
There is no shame in not liking to get hit very hard, not having much money, or just simply preferring those kinds of weapons. Of course, if you like vulcan miniguns and use vehicles and have games that last a weekend straight including camping, that's another story.
What am I using my gun for?
If you like to do target practice in your room, that's fine, and there are some good instructables for targets. If you are playing with your friends, there are several roles you can play.
Leader/Auxiliary You will want something automatic (battery, rapid fire) that can lay down the plastic but also shoot accurately-ish. Any assault rifle looking gun should work.
Sniper/Designated Marks(wo)man Obviously, something accurate is in order, and a gun that looks like a sniper rifle should do the trick (airsoft guns tend to look like their real counterparts in terms of functionality).
Support Gunner get something that can lay down a wall of plastic quickly. A support gunner uses lots of ammo and something called "Spray and Pray" tactics because you spray at your target like nuts and pray one or to will hit. Miniguns, a SAW style gun, or, in a pinch, anything with a decent rate of fire should work.
Remember, you can have more than one or none of these among your friends, as these are simply the basic roles requiring different guns.
How much dinero have I got to burn?
If you don't have much money lying around, you can still get a pretty decent weapon. (As long as you don't take up paintball!) My UMP was $110 and is very good. Decent weapons can be had for much less, however, so fear not. You won't be getting the same quality, guarantees, or functionality, but most who spend a lot on a new gun get only maybe 15% petter performance anyway. (If you are about to comment that your $1500 custom M4 is waaaay better than anyone else's and it's soooo worth it, maybe it is, but I dont want to hear it and neither do the newbies who just want to have fun with their friends without being intimidated.)
How many/ what capacity magazines do I want?
Magazines (or clips) are containers that hold your ammo and push it up into your gun. (Wal-Mart plastic-fantastic guns may not have these.) If you are a sniper, you are in no rush and will not use up much ammo, and will not need too much. (One or two low-capacity clips should suffice. Remember, bring extra BBs.) If you're a support gunner, your clips will be sizable but you will still want a few. For Leader/Auxiliary, you will want a few clips of high capacity (if you don't like reloading, like me.) Or you could lug around the extra plastic of many low capacity magazines.
What FPS range will I be playing at?
This is a big question. FPs is an acronym for
There are many factors that would decide this question. First, how much do you want to wear? A sweatshirt should more than suffice for under 200FPS, but more than that and you may want to wear something extra. Also, don't worry too much about your sensitive areas. Almost always, the cloth/folds in your pants will do the trick. Some wear cups, but that is overkill since the projectiles have little momentum because they weigh in at around a fifth of a gram. Paintball is another story. For the ladies, a padded sports bra should suffice, but I lack the equipment to know this for certain, although I'm guessing that the projectile needs more weight to have enough momentum to hurt there more than any other part of the body. Feel free to correct me on this.
Also, at what range are you playing at? With an aresenal from a department store, 20 feet can be an accepted range, but with high FPS weapons, that same range should be a no-fire zone.
What do I want it to look like?
Personally i like neither of the common types of airsoft guns (AK family and M16/M16/AR-15 family), so I have Heckler & Koch MP5s and a UMP. Just a matter of personal choice.
Once you have pondered these questions for awhile, go to the next step.