There are many variables to the accuracy and range of airsoft guns. It depends on type (springer, GBB, AEG, ect.) and upgrades. It also depends on barrel length, weight of bb, conditions, as well as how good of a shooter you are.
You must remember that a shot will drop and thus account for it in your aiming. Also because we are using plastic bb's wind plays a huge roll.
For my stock CA M15A4 I can make a shot probably close to 50 yards with no wind using .20 bb's
Rules of thumb are simple:
-More weight BB's = More drop, less wind movement, harder hit, less range,
-Lower weight BB's = Less drop, more wind movement, lighter hit, further range
-Tightbore Barrel = Better accuracy
-Longer Barrel = Better accuracy, tighter grouping
-Shorter Barrel = Spread grouping, lower accuracy
It also depends on your playing style and position.
Sniper - Upgraded Bolt Action or longer upgraded AEG (M14 ect.) For longer range. Usually between 400-450 FPS (semi only for AEG depending on club)
Rifle Man - Medium Distance Weapons such as M16, M4, Aug, L85, ect. Can be upgraded. Usually 325 - 400 FPS (depending on club)
SAW Gunner - LMG style gun such as M249, MG36, M60, ect. Usually between 325-370 FPS (depending on club)
CQB - Shorter, close range weapons such as MP5, P90, UMG, MP7, M4, ect. Usually 325 - 350 FPS (depending on club)
My best suggestion to you is find a club in your area. then pick a position for gaming/team. Do you like to shoot lots, advance maneuver, supressing fire, sneak and stalk, call targets, ect. I started out with a MP5 and decided I didn't like the close quarters stuff so I bought a M15. Point being find out what you want to do first.
Then examine what your budget is. More money = better gun. Also keep in mind cost of kit and upgrades
Next pick out a gun that is comfortable and can do what you want it to do for your desired position.
Next step is research who makes better guns, which last better and hold up for what you will use it for, whats out there for parts, ect.
Then buy the gun you pick out, buy the kit, find a game, and go shoot people.
Also once your 18 get age verified buy your local rep. There are lot's of good guns for sale in the classifieds. Your in a good position right now because you have lots of time to research kit and guns, as well as ask around.