Basically there are a number of airsoft sites around the UK, so best bet for having a go is to find one of these in your area, get in touch and get a hire gun booked and then go and play and have fun.
Sign up to the airsoft forums and have a read, most of your questions be answered there...but in brief airsoft is similar to paintball but instead of paint you shoot a 6mm bb and use a realisitc weapon (looks, feels and in some way operates like a real fire arm)
Anything specific you want to ask feel free to, a number of us airsoft, some good forums are
Arnies AirsoftAirsoft Communityand we have a member here (Redcap) who has a good website
Practical AirsoftTheres a team forum listed as SGC UK on Airsoft Community where a number of the UK SG teams have congregated...and we from SG-4 are in the process of setting up a team...called SG-4 (genius arent we)