If you're going to play on a paintball field then for insurance purposes the field will usually require that you wear at least paintball goggles. If you're not on a paintball field then ballistic goggles such as ESS are great. Shooting glasses will work in a pinch but are not recommended since bb's can possibly get in on the sides.
As to some sort of facemask, face bleeders happen, and chipped teeth are possible, but alot of airsofters take the risk. Almost no one uses hard masks like paintball masks cause they mess with cheek weld but some use soft balaclavas or wraps or such. The reason why alot of people don't even bother the balaclavas is it's hard to talk and be understood in the midst of battle with those. It's a judgement call on your part. The problem with mouth guards is you can't talk. Hard to coordinate tactics when you can't talk. No one I know uses them.