Pretty much what duck said. Don't buy from any website with bbguns in it, because generally they charge way over the price, lie about the stats and then when it breaks you will never get the returns policy. Main sites to buy from =
Proairsoftsupplies.com
zerooneairsoft.com
landwarriorairsoft.com
patrolbase.co.uk
all of our sponsors and people like armysurplusandtoys and ak2m4 who are on these forums, they can generally give you a forum discount
also tawain gun and there are a few others like JD that are ok, not brilliant...
I bought off of zero one and you have to do a lot of paperwork for your first order but it is fine after that, just for safety precautions and everything, its good its not just a box to tick because that doesn't clear that you actually are 18 *cough*justbbguns*cough*
My first proper gun was a G&G cm16 carbine, I got the carbine because I liked the idea of changing the stock and I just generally like it more, but personal preference seems to be most people like the raider so I imagine that will be you.
I got the long, however the short would be more practical for me in terms of carrying it around, because I cycle to mates houses for garden games and you have to be careful about hiding so I have to carry a huge backpack on my back which fits it, ( a hiking one) and it's tough to cycle with.
Battery wise - get a LiPo. I got the 2600mah off of component shop and it is a beast, also with the accucel digital 6 with power supply, you spend around £70 altogether however in the long run it will save you. LiPos are great becase they give more power fore less, last longer, are smaller and you don't have to discharge them, just keep topping them up. They don't go wrong if you use them correctly e.g plug in the balancer, and use a smart charger so they don't go over max charge. That charger has an LCD display, it can charge from lead acid to LiFe, it is great and I use it.
Eye protection is something you don't want to go cheap with. if you are under 16 you have to have full face and a good bolle mask and a half mesh mask could set you back some money but could save your eyes. Don't get the cheap chinese ones, in case something does go wrong, it could be serious and you would then regret it. Paintball masks work fine as well, but they do get fogged up like any mask or pair of goggles, you just have to be careful.
Clothing wise, indoor is fine to wear chinos and a top or whatever, doesn't really matter about camouflage, however if you are going to play woodland I would advise you to go down to your local surplus store and buy some old issue woodland kit or multicam, whatever fits you, but it will make a big deal rather than wearing bright green like I did on my first skirmish...
Magazines and chest rigs wise, for your first few games I would carry a rucksack around with BBs in, put it down to play and refill your mags, after, thats if you have 2 or three mags. Depends at what site you are playing, with mine they don't mess around, 2 minute break so we don't go back to the safe zone, so I carried some BBs around which saved me a few times.
I have probably missed some key aspects, and I am worried I am becoming a teacher with my essays, but that is really all I have to say, will add some stuff if there is something else I forgot to mention in this post