Good luck finding a game since you're under 18. The feeling and responses I got from the majority of the people on this group is 18 or older.
As for gear, I would suggest the bare minimum of boots, BDU's, Camelback (or other hydration carrier), head cover (hat, etc) and of course safety goggle or mask.
You want decent footwear, especially boots that will support your ankles. You can get a decent pair of Brazos Service boots (style like cops where) at Walmart around $30. Those are good for casual use, once or twice a week. I would suggest you wear them continuously for about a week to break them in though. If you want something tougher and that will last longer look at West Valley Uniform (91st ave/Peoria) for a full line of tactical boots. Also check out Allied Surplus.
I say BDU's because they're designed for tactical use, have deep pockets, easy to move in and will be more comfortable in the long run. Any surplus store will have these. You can also goto West Valley, they'll have BDU style pants and tops in blacks, blues and tans aplenty. Also check out
www.bdu.com for a variety of styles ranging from $15 to $50.
A key thing to remember ESPECIALLY in the Arizona heat is hydration. Having water bottles nearby is great, but you can't beat the convenience of a Camelback or other such hydration carrier. You can get these at most local sports stores. Cabellas on Glendale across from the Hockey Arena is a good place. Cabella's also may be a good one stop place for many of these items. A hydration carrier keeps you water with you on your back, makes it easy to take a sip while on the move. The convenience and ease helps you stay hydrated since your more apt to sip from the tube rather than stop to retrieve a bottle, unscrew it, sip, then put it all away.
A hat or some sort of head cover is suggested. It keeps the sun off your head, face and depending on the style your neck. This cuts down on heat exposure and sunburn.
And of course safety goggles or mask. I shouldn't have to go into detail here.
That's pretty much the basics for a good start to gear. As you get more involved, change up your weapons, go from hi-caps and box mags to lo-caps and mid-caps you'll want to check out vests and pouches.
Being that you're under 18 you probably won't find many other groups or fields that will accommodate you. Marauders Tactical (
www.marauders.us) allows minors with parental presence. They only have to be on site, not necessarily active in the game. The above gear/uniform suggestions I made meet their minimum field requirements with the exception of the full face masks they require.