For you first few games a dark coloured hoodie and some old jeans will be sufficient, but the big attraction of airsoft for many people is dressing up in cool kit and generally looking like a badass.
Shallow? Probably. Fun? Definitely!
You can obtain the equipment and uniform of practically any army in the world, past or present, if you are willing to pay for it. You can even have your kit custom made. There are always sheep-like uniform trends which come and go but you are not obliged to wear anything in particular unless you go to an unusual scenario day where you have to represent a particular fighting force.
You may be tempted to get a full set of uniform and kit before your first game but you may then find that your local site or you play style doesn’t fit what you had in mind, so it is much more sensible to get something cheap from an army surplus store and upscale your gear later on.
The best stuff you can get in the UK is British DPM. It is cheap, hard wearing, good camouflage and comfortable. The market is flooded with excellent quality DPM due to the introduction of MTP to the British armed forces a few years ago. You should start by getting a shirt or jacket and a pair of trousers. If you are not buying a set of load carrying equipment (LCE) look for a jacket with pockets that you can put your spare ammo and magazines into.