We get a lot of people coming along who want to know what's best to get when starting out. For those of us who have been around for a while, it's sometimes difficult to avoid being sarcastic, due to the ubiquity of certain misapprehensions and bad ideas. To avoid repetition then, my two pen'orth is that, in this order, you should:
Read the Relevant Legal Information: Violent Crime Reduction Act, as it relates to us, here.
Buy Boots. If you are young I'd guess you're used to wearing trainers. You need boots that support your ankles so that you can trip over or step in a hole while you're running and not break / badly sprain an ankle. They also need soles tough enough to withstand woodland debris / urban rubble. German Para Boots or British Assault Boots are you're best bet, but your feet will be too soft to wear them, unless you get some and go for long walks in 'em to toughen up, and even then you'll want to use tape to prevent blisters if you only wear them a couple of times a month. Magnum Panther 8's should be ok for your feet and tough enough for most airsoft sites, but bear in mind that they are water resistant, not waterproof like most Military Surplus boots. You can get some great deals by visiting military surplus shops in person too.
Buy Mesh Eye Protection & Lower Face Mask. There are a number of issues involved, but suffice it to say that polycarbonate lenses often fog up, so a full face mask like this provides the best average between safety & see-through-ability. However wearing one of those means you can't get anything in your mouth without compromising eye safety and causing site marshals to go nuts at you. Hero Shark are currently your best bet for eyes, but the website is down so I dunno where you can get them (see below), otherwise get interwoven mesh like these. For lower face, to prevent you losing teeth and/or getting permanently scarred, get a New Version Strike mask in whatever colour suits your loadout best from KH Mountain.
Go to your Local Site and Rent a Gun plus, if possible, Camo Gear - otherwise just wear jeans and a hoody / long sleeved T-shirt if it's hot (to prevent BB's breaking the skin of your arms). While you're there talk to players about their guns and gear - most people will be happy to tell you anything you want to know and let you hold their guns, shoulder them, etc. Try to have a hold of one of each of these types before you decide which assault rifle suits you best:
M4 
or 
AK47/74 
or
G36K/C
MP5 or
L85A2 (SA80)
Buy Surplus Camouflage Clothing from eBay, stuff like these & these. Do not pay more than £20 for both inc P&P, but you should be able to get them for less.
Buy Plastic Armoured Gloves - they may not look the coolest, but they are the best protection from BB hits to the hands (which hurt). Otherwise get Hard Knuckle's for looks or Viper Tactical Gloves for winter.
Join an Airsoft Site and, if you're over 18, get UKARA registered. If not, your membership still proves that you are an airsofter and some retailers will take that as defence against prosecution under the VCRA. Bear in mind that u18's cannot buy any airsoft gun, RIF or two-tone IF, but it is not illegal for an u18 to own a gun without hideous two-tone paint. Some retailers will sell a RIF to an adult accompanying a minor who has proof of being a skirmisher, but it is also not illegal for an u18 regular skirmisher to 'manufacture an RIF', ie strip the two-tone paint off (with Fairy Power Spray). Therefore, do whatever you need to do to get a Realistic looking Assault Rifle. The G&G Combat Machine range is probably your best bet, from Zero One because they'll two-tone it for free. However CYMA are better for AK's and SRC for G36's.
Buying Batteries and a Charger is often best done with the Gun, to avoid further P&P charges, but otherwise go to Component Shop and buy Vapex (VP) batteries. A 1600mAh battery pack will usually see you through a full day, but it's best to have a spare of at least 1000mAh also, for those heavier trigger finger days, or just get 2200+. Still, things do go wrong so a spare battery pack isn't a bad plan, whatever power your primary battery is. NiMh batteries can be charged from a trickle charger and these are available with auto-cut-off for less than £10. LiPo and LiFe batteries need a smart balancing charger and these can be had for less than £20.
Buy at least one Spare High Capacity Magazine with your gun. Later you may decide to swap to mid or low cap mags, but a couple of hi caps will still be useful as back up. MAG brand have a good rep (see below). CYMA make good AK mags. Classic Army or Ares for G36 mags.
You will also need something to carry your rifle in so it cannot be seen by the general public on your way to and from skirmish days. A British Army Duffle Bag is probably the cheapest option and you can get three rifles in there, or one and a bunch of other stuff. If the muzzle sticks out of the top by a few cm, do what I do and put a canvas bag over it, or a sock would do lol! There are cooler options, Swiss Arms rifle bags for eg, but you'll have to pay accordingly.
At this stage the whole plethora of accessories and gear will seem very tempting and I don't think it matters much what order you consider / get them in, but a lot of stuff is ultimately useless so I'll just mention things which do help:
A Vertical Foregrip is more comfortable to hold than the horizontal handguards of a rifle.
Knee Pads make crawling through undergrowth / kneeling in mud easy.
Likewise Elbow Pads, which also make aiming while lying prone in difficult terrain easier.
A Brimmed Hat keeps the sun out of your eyes, but most Helmets do not, so although they are much cooler, you may then have to rely on Dark Lenses in your eyepro, which will fog when you get hot and sweaty from running about. There is another option which is to get Goggles with perforated sheet mesh, like these or these, because they darken the view like shades.
A Red Laser is pretty useless outdoors, except at night, and a lot of sites will not allow the more powerful green lasers, but Red/Green Dot Holo/Reflex Sights can be good, so long as they are bright enough so look for one with variable brightness settings.
A Hydration Pouch & Bladder is such a good thing. You will get a dry mouth due to breathing heavily and also lose moisture by sweating. To be able to simply take a sip whenever you feel like it is brilliant compared to messing about with a canteen which tends to be much less comfortable to carry as well as being more hassle to get out and drink from.
A Dump Pouch for empty mags makes reloading faster than if you need to replace the empties in pockets or pouches. A Drop Leg Dump Pouch is best IMO.
If you are going to carry more than two spare mags, you will want some kind of Assault Vest, Webbing, Chest Rig, or Plate Carrier. Molle is most versatile, but expensive compared to surplus items.
This Bulle MOLLE Recon Chest Harness is a cheaper, lighter, alternative to a full Vest or Plate Carrier, but remember you also need pouches for it.
British Army Northern Ireland Chest Rig's are the cheapest - on eBay they auction at 1p + £6 P&P
.
PLCE Webbing is pretty cheap, around £25 posted from eBay if you're patient
and it's indestructible but also heavy.
A British Army Assault Vest
is a good all round choice and you can get them for around £20-25, less sometimes, also from eBay.
My advice would be to buy some alternative clothing that you choose for the look and a Back Up AEG before buying a Pistol, but hey ho, they are good toys...
You do not want a Sniper Rifle or a Gas Powered Rifle / Sub-machinegun
until you have been into this sport for at least a year. Bear in mind that, on top of the price of the gun, it costs a minimum of about £250 to upgrade a sniper rifle so it's worth taking to a skirmish. GBBR's take more maintenance, hence knowledge, and the mags cost about £35-40 a piece...
All comments, discussion, disagreements, questions, etc. welcome. Have at it