what to learn?

grab your guitar, put a CD in the stereo, find a song you like, hit play, and then try your best to play along.
that is the kind of pressure you will feel when playing with others; thinking fast.

replay the CD over and over and practice on it.
I think it is a great way to learn a few things.
like listening, understanding song structure, and you get a feel for tempo, timing, and phrasing.
plus, you are not playing in a vacuum.

BTW, those are terrific goals.

It think depends on what sort of band/music you are wanting to play, and what type of guitar you play as well; also wether you want to be rythm or lead guitar.

These are the things that I personally have realised from learning over the past few years;

If the band you are looking to join plays a lot of soft-rock, acoustic-ballad type music (things like Bryan Adams, David Gray, Ben Harper) then skills like forming full barre chords and fingerpicking / arpeggio's are going to be useful, as is having an acoustic guitar.

If you are looking to play mainly punk/rock (Blink 182, Greenday, Offsping), then electric guitar & power chords are going to be your main base, with some soloing abilities perhaps.

If you are looking to play hard rock/metal (Mudvayne, Trivium, Killswitch Engage), then electric guitar using dropped tunings (drop D; DADGBe and drop C; CGCFAD) are going to be a big part, as is a somewhat faster speed of playing.

Of course these things are NOT the only skills you will need, and in an ideal world you would learn all of them regardless of the band's "type"; after all, a rock song can be transposed onto an acoustic guitar with really great results (Going Under by Evenescence is great to play on the acoustic despite the orginal version using a drop-D electric guitar for heavy sound).

That's all just to give you an idea of what to expect (and I could well be corrected by other's who have differet ideas).

Also Dogbite has good advice, just start playing as many songs as you can learn.

As for writing for the band (not sure if you meant music, lyrics or both) again, the best way to do this is simply to just start doing it. Write a bunch of lyrics (even if they are not too good) and get going, you'll get much better with practice. Join the SSG forum here and try to write a song each week. Play around with chord progessions, see what works and doesn't, and record yourself playing (it's a great way of reflecting on your playing). If you find a progression / strum pattern you like when listening back on your playing, make note of it and try to develop it further.

If I ever grab my guitar for anything more than just running practice drills, I always just hook it into my laptop with a cable and record everything I play - that way, if I come up with something on the spot, I don't have to worry about not remembering it later on, as it is already recorded for me to listen back to.

Same with lyrics as well, write anything that comes into your head, and keep EVERYTHING you write. Looking back over old songs is a good way of seeing how far you have come, and can also give you inspiration to write the next song (or even to improve the old one with your new skills).

Hope that's all of use!

Pete

You haven't mentioned what level you are at now. How about talking to the guys/girls in the band you want to join and ask what they need?

I'd guess he's at this level?
https://www.guitarnoise.com/forums/viewtopic.php?t=32471

lessons might be an option :?

I'll be 100% honest here. You've had your guitar for like two weeks now, you can't really be in a "BAND" now. Go ahead and jam with your friends but only if it's teaching you something. At this stage, you're just dreaming.

Sorry if that came across as rude...it's just true.