Ummm I was going to say mask is the first thing you should get. (But you already know that.) I personally bought a 2nd hand Proto from gumtree for £5 and it's fairly comfortable and the lens is a tad scratched but for £5 ... no bad! I would suggest, however, making sure you get a mask you like and that is comfortable. For example maybe a Dyei4. (More experienced players can probably advise you better)
After that, I think, it's personal prefference. The advise I'd give, which I ignored, is to play a fair bit before buying a marker and remember that you don't JUST need the marker you need a hopper (There are a few different types, spire, rotor and gravity fed?) which again is personal preference (Id suggest a rotor), as well as an air tank, which again is dependant on the marker? and dependent on the size will make the marker heavier or lighter.
Oh and also if your gun doesn't come with a barrel sock you'll need to get one of those and maybe if you want? A cover for your air tank. (Oh and a squeegy to clean your gun)
Other kit I have personally: Guess it depends on your preference, how much you're going to play, what kind of paintball you'll be playing?
+ Headband
+ Bounce vest (Only padding you don't have to have it)
+ Elbow pads (that go all the way to my wrists)
+ Knee Pads
+ Trousers (They'repadded also)
+ Jersey (Guess what ... more padding.)
+ Gloves (Because being shot in the hands hurts enough; nevermind on bare skin.)
+ Beanie Hat (Personal preference to stop my hair getting paint in it.)
I'm also going to be after some slide shorts and maybe some football foots.