If you are concerned about really serious safety issues (not having bleeding skin ou marks on your skin) then you don't need much. Goggles and something to protect your teeth is pretty much all you will need. Wear thick clothing (no t-shirts and such) and you'll be fine.
I use a padded half-mask (like what snowboarders wear). It protect my neck and teeth. Not as safe as a full face mask or mouth protector but it does the job. As stated, you can also use a full face mask (safest) or a mouth protector like boxers (small but harder to speak with).
As for your back, there isn't much you can do. BBs won't hurt your back, falling or tripping will.
helmet : more or less useful. BBs on the head can sting a bit but nothing serious. I played for long with just the equivalent of a bandana on the head. Many play without anything on the head so a helmet with ventilation holes isn't a problem.
Elbow pads : have some, never turned out very useful yet except for crawling
Knee pads : useful but not often. Not a bad idea.
Cup : don't have one and will not get one. Never had a problem there. Might regret it some day but the pain will pass
Gloves : very very very useful. A BB on the finger hurts like hell. Not essential but a great buy.