I know that you most likely know this by now.. But eyepro is the most important thing. You only have 2 eyes, if you lose them, you won't be popping into ASDA to buy a new pair.. For clothing - I used to want to get proper military clothing, vests, helmets etc etc - but, all I run is these Camo coloured trousers, in the colour of dark greens and dark browns. Some hiking boots, some t shirt, and a green fleece from an outdoors shop. You can carry mags in trouser pockets, but some prefer not, so they invest into a vest with pouches.
For mags, I'm a scum bag and I use a high cap (they came with the gun)
On a (so very) serious note: you'll be wanting to carry atleast 5 high caps, as when you see the enemy, you'll be wanting to let rip all 450 BBS in your magazine, as this is quite clearly the only way to get your target. (I forgot, when you've shot 40 BBs or so you'll have to rewind your mag, so, well, you may look like a plonker, deciding to be like " rahhhh " then "oh sh*t, wait" it doesn't matter though, just rewind that sucker, and then let rip again. Spraying all your BBs across the map. And also, if you want this to work, you'll have to buy a vest with 10 pouches, if you own an M4 you'll be wanting AK mags in your pouches too, just to look like a l33t 0p3rat0r. And one of those legitimate gun sound emitters are quite obviously necessary for airsoft, as airsoft isn't a game, it's war right? Oh, also, a reason to have high-caps is because, they make a noise that will just annoy everyone, but also gives away your position, it may give away your position, but it results in more enemies coming closer to you, therefore, you get more airsoft kills. ugh.
On an actual note, people will say that you have to wear vests to be cool/good/ a l33t 0p3rat0r etc, well they're wrong. Don't listen to them, it's more of a "I like that thing, I'll buy it" not a "this will let me murk everyone on the field", vests and accessories (I like to call them) is your choice, it's not a requirement.
Just enjoy the game and have fun, that's the best way to play airsoft.