first off, welcome to ASC. first step is to get age verified and fill out your location. from there, you can look on the classifieds for accessories. as for batteries and bbs, it depends on a few factors.
- what is the weather like?
- are you trigger happy?
- what is the fps of your rifle?
the colder it is, the faster your battery will drain. just as well, a higher fps will lead to a faster battery drain. from personal experience, i bring 2 batteries to a game. you may need more, you may need less.
the amount of bbs you use also depends on your play style, if you rely on pray and spray bring a s**t tonne of em, if you are more controlled, that bag of 2000 bbs might last you a couple games.
as stated before, this is just from my experience. your best bet, go to a scrim and get a feel for it. the more you play, the more you learn. try to go out to the field with someone who has been in the hobby for a while. this is my second season here and its great. keep a level head, and watch out for the old guys. there bark is worst then there bite XP