Your typical hire is about £50.
That covers everything. Gun, Camo, Eye protection, and normally about 3000 bb's.
Extra BB's are normally £8-12 for another 3000.
Some sites provide a free lunch.
Most sites provide free tea and coffee.
Soft drinks and snacks are usually extra.
If you plan to run take hydration. You will sweat even more if you are in full kit, with a helmet. Most sites are pretty small, and you will get killed often. Every kill you tend to head back to a respawn location and run back into the game after a short countdown. It means you can spend a lot of time on the move and it'll take it out of you.
If you already have shooting glasses then make sure they are rated to take impacts (they should be).
Teeth can get shot, and do get smashed. If you don't want to go to the dentist then lower face protection is advisable. Mesh is fine, You can get all in one eyepro and lower face protection, but don't get the cheap ones. If you want a decent set have a look at the Dye i4 and i5 masks.
Book a day with the full hire kit. It doesn't matter if you are alone, Airsofters are friendly, and you will probably find some ex-services or cadets there anyway. The Marshals at most sites will also look after you. Learn their names and spend some time chatting them up for site information and where the good locations are.
Chat to the players about the guns and gear. Most of it you will know but airsoft isn't like the real thing. The guns have no recoil, are lighter, and have significantly less range. If you chat up the walts in the safe zone there is a good chance a few will let you look at the guns and have a shoot. Been friendly will get you very far, most of us think we have the best gun ever made and will tell you all the good points about it as if it was significantly better than everyone else's. Generally, the guns are pretty evenly matched and everyone is energy limited by law and site rules.
Equally, the guys with the best gear might not have a clue on how to move on the battlefield. This is a game of open wallets, and because of that some of the best players are the ones in jeans and a T-shirt with a gun that looks like it has been built from scrap.
Make sure you ask a marshal to set up your "hop unit". It's a bit of rubber in the barrel that puts backspin on the BB. Having it set right will give you better range, A lot of hire guns will just be thrown in a corner and won't be set up from the last 13year old fingering the moving bits.
It's also worth having a small pack of wet wipes. Some sites are pretty shy on amenities. And running water can be a luxury they didn't pay for! I've never seen a long drop, but there must be a few sites with them! If they have porta-loo's don't expect them to be clean. Some sites are fantastic the one I usually go to even has showers, So just be prepared literally for anything!