If you are going to buy a new mask, buy a new lense for it straight away. You don't know how well the mask has been looked after.
(More precisely - Unless buying a new mask get a new lens)
First thing realy is to make sure you play and look around at what everyone else has and you can ask for a look and advice. If you play at a normal site you can rent all you need for the day. Even if you go to a walkon you can rent for the day, (but call first to make sure if it is an exclusive walkon day)
Regarding what to wear, start with comfortable clothes - loose top and bottoms. If you go to a normal punter site they will supply camouflage overalls. Wear something light and loose to go on underneath. Bring a change of clothes for the way home. I prefer not to wear the overalls so would always haev my own gear. It can be camouflage or just your average clothes as long as you odn't mind running around, getting shot in them and possibly falling over and ripping them.
Think of your footwear - are you going to run around everywhere, dodge back and forth or hide and sneak? Trainers are fine.
The rental gear you want for the day are mask, marker, hopper, air, paint pods and a pod belt to carry them. Plus you will need some cash for paint.
When ready to buy a decent mask is what you need to look for first. They can be cheap or expensive. Sites often to have for sale the cheap VForce rental goggles. They are decent and cheap, with the benefit of you having a nice new pair of your own goggles on your first day out.
The second thing to think of buying is your marker. First try to check out what everyone else has, then think of what type of paintball you want to play. Don't blow all your money on the first marker you see, and look at the classifieds for a second hand bargain.
Of course you will need air (or CO2) The option will be dictated by what the sites you intend to play out supply. Air is preferable.
You can spend £150 on a good 4500psi air system, but for starters a £40 steel 3000psi bottle will do you fine. Ask at the site when you are there for the first day and they even might sell you one if they have new ones in stock. It's also possible to bring your own marker and rent/borrow a bottle from the site before you buy one if you decide not to bother with the 3000 at first. (Note you cannot always get 4500 fills, normal games don't last long enough to drain a 3000 bottle - so I recommend leaving the 4500 bottles for a later buy)
Somewhere along the way get a reasonable electric hopper*, pods and a pod belt. (* Unless your choice of marker makes the choice for you, e.g. an A5/X7 cyclone fed marker requires a different type of hopper which comes with the marker!)
Fingerless gloves are useful too.