So the first thing you do is see what kind of band you like. Rock band? Heavy Metal? Punk? This will help you decide how many members you need. I wanted a Hard Rock band, so I got another four friends, plus a backup or two. If you want something more punk, then two or three friends should be enough; one guitar player, a drummer, a bass player and a singer. Rock? Two guitars, vocals, drums, and bass. Keyboard is optional. Same goes for heavy metal.
The next thing you do is see what you can do. If you're simply good at managing people around, then maybe you don't belong on the stage. Still, there are some members in rock bands that can manage them, for example Lars Ulrich, Metallica's drummer. If you play guitar and can rock some mean solos, then go for lead. If you play a good rhythm and don't get lost in time (as many lead guitar players do) then go for second guitar.
Now, for guitar players, assuming you don't play guitar, you will need one who can play solos and one who can play rhythms. They have to be able to sync with each other (I was lead guitar and I could never sync with the other guitar player), and have their own chemistry, asides from the general band chemistry.
Finding a bass player is tricky, as not many people play bass, and not many are willing to get in a band. Also, he has to play with the style you want. If you want to play heavy, then you need a fast player who plays with thick strings and maybe even picks. You want funk, then find a bass player who can slap it. Punk, then he just needs fast fingers.
The drummer is the most important part of the band, he's the backbone. If you can't find a good drummer, chances are your band won't be worth it. Why? He keeps the beat, rhythm, mood, etc. My drummer was the best I could grab. Why? Because he knew how to play everything. Punk, rock, hard rock, heavy metal, grunge, you name it. He could conjure up a cool rhythm in minutes, to follow the riff I'd created. He was exceptional, man. You need one who can actually play, and play well. If he sucks, then the band sucks. A guitar player can suck, damn it, lead guitar can suck, but not the drummer. Same goes for the bassist, he has to be precise.
Now, this one's a bit easier than the others. And still, hard. The singer has to be in sync with everybody, and have good chemistry with everybody. Why? He's the frontman. He's the image of the band. Axl Rose was Guns n' Roses, Jimmy was The Doors, Cobain was Nirvana, and damn it, Ozzy's Ozzy! The singer has to be able to keep his vocals in range, not too high, not too low, never get out of key, be able to run and sing (for stage presence) and write his own songs. Why? You won't feel as good singing somebody else's song, it needs to be your own.
So, since summer's coming, then you'll have a bunch of time to try all of this. Maybe you'll become a hit, who knows? The thing's to have fun with it. So, go out there, make a band, and see where fate takes you. At least you'll have a cool story for later on, huh?