Well, if you want to start out basic, You can usually get a single shot CO2 paintball gun at some hardware stores / super centre's such as wal*mart for pretty cheap.
If you're really set on it, Invest in a High pressure air (HPA) E-marker. They require batteries to run (electronic boards). They are expensive but can shoot around 13+ BPS.
As for extra's, It's usually best to go online and buy a pair of padded paintball pants, They not only lessen the impact but will help if you need to kneel or go prone. Along with that, get a jersey that's 1-2 sizes larger then you and get a padded vest to fit underneath.
As for masks, try to get the lightest one you can. Less strain and better vision.