Step 1: You sure?
The first step is important, and that's if you're sure you can handle shooting people (possibly your friends) and being shot (again, by your friends). I recommend trying the sport out first, renting before buying, and seeing if you like it.
BEFORE YOU PLAY:
This sport does sometimes draw blood, cause bruises, and definitely raise some welts. At times I have experienced a mouth full of bb fragments, a bleeding neck, ear, arms, blood blisters on the fingers, and yes, occasionally welts on my butt. If you can handle your coworkers / spouse / fellow churchmembers seeing those battle scars, and perhaps wondering where that hickey or strange skin disease came from, read on.
Airsoft is about realism, they are called airsoft "guns", not like paintball "markers" for a reason. Airsoft guns are made to look as close as possible to real steel weaponry, and advanced play defines Airsoft as a "milsim" - military simulation. Systema for example is an airsoft manufacturer that is geared more towards "training tools" than "toys". Remember that running around with a replica gun will get you killed, arrested, or slowly choke airsoft as a legal sport in the US. Most players of all experience levels would attend an organized outdoor event before finding some willing private property owner that would allow them to destroy their land with plastic bb waste, and bb shredded trees and plant life. If you live in a rural area, and you just want to skirmish with a few friends, the least you can do is inform your neighbors if you're near their property, and local law enforcement as well. Remember too that plastic bb's will be there a long time, there are some biodegradeable bb's marketed, but from most research they leave much to be desired.
Backyard airsofting in the middle of suburbia is ALWAYS A TERRIBLE IDEA. Helicopters, low flying planes, people passing by will see what looks like either a small scale shootout or a homegrown training site for rebels-terrorists-whatever. Just don't do it.
I've seen a few other places where people swear by backyard airsofting, but in my opinion you're still asking for trouble, and you're just lucky that people have ignored you so far. Getting caught is the proof, and who's going to come back and comment about the time you and your friends were heavily fined, arrested, and jailed for backyard airsofting? No, the only commenters will be the ones who haven't been caught yet.
FINDING A PLACE TO PLAY:
Okay, finally! Here's where this instructable really starts. CQB or CQC - close quarters battle/combat, is I think the best way to jump right into the pool.
Find a good CQB airsoft site, next, call the site! If you're a total beginner there are a few questions you need to ask:
1. Are there rentals, supplies available for purchase onsite?
Can you rent a gun and/or a mask there? What about buying gas, bb's, snacks/drinks, maybe even a gun if you want. Rentals are almost 100% sure to be available, but you might as well be sure.
2. What is the policy regarding eye protection?
Some sites may require full masks, while others require only approved goggles.
(Note: Approved goggles do not mean shop goggles, or even shooters glasses. Shop goggles will shatter into brittle shards, and shooter's glasses do not offer fully closed protection. Even military spec'd rain/wind/dust goggles will break, you need to find a good paintball mask or purpose-built airsoft goggles at the least.
3. If you're a minor, what about parental consent?
Do they have to be there? Do you need any ID?
4. What are the hours?
Airsoft is still a relatively small and niche sport, sometimes my friends and I will call up simply to ask if there's anyone there at all. (Don't get me wrong, a small game can be just as fun as a wild 30 on 30 shootout.)
5. Do they take breaks?
This might be unique to the field I frequent, but they take a 30-60 minute break for lunch and dinner (wristbands show who has paid and can reenter the field), after the dinner break (~7pm) prices are cut to a half day rate, so this might be a good time to jump in.
So now that you've called and you're ready to leave, how do you prepare?