I'm a seventh grader and I want to learn how to play bass, and while I already have a guitar I need to learn how to play. I recently got invited to a friend's band because they need a bass player and he saw that I had one and know I don't know what to do, I really want to be in his band but I can't play. Can you help me?
This is a very broad question and for that it will be difficult to help. Does your friend know that you can't play yet? There's nothing wrong with learning as you go. You say that you have a guitar and that your friend saw that you "had one". I'm guessing that means you have a bass too. Is that right? The best thing you can do is take lessons somewhere. If no other option exists look up some beginner lessons on line, you might find some free stuff on you tube. But the best thing is to get a real instructor looking at your hands and guiding you on correct technique. If the style of music is not too difficult you may be able to pick up a song quickly using TAB. I guess the real question(s) is (are), do you want to do this, and does your friend think you already know how to play but you don't? You need to be honest with your friend.
It's quite often the case that bass players were guitarists in an earlier incarnation!
Quite straightforward - the four bass strings are each an octave lower than the bottom four strings on an ordinary guitar!
There are no chords to play (in mainstream), so even playing a 1 on 1 - root note on beat 1 of each bar - will get you started.
If you have a good ear, you can probably pick up what the bass plays in each song, and copy it, but it's in your interest to know what you're doing, so lessons won't go amiss. Leard about keys, and which notes make up arpeggios, and which notes will get played most in particular keys. Learn how patterns work on bass. Learn how to be a team with the drummer, listening to each other, and playing so you complement each other.
In my first group (I was 14), the bass had to use an ordinary guitar, so you have a head start!