Read Chapter 1-Forming a Band
After you have a band together and the basic idea of where you want to go sound wise, you have to practice. This is where 90% of bands fall apart. "Yeah, man, let's practice Tuesday at 7," and then Tuesday at 7 comes and nobody shows. Joey's fighting with his girlfriend, Gabe is stoned, and Jason just forgot. Repeat a couple more times and nothing ever happens and the idea of the "band" fizzles out.
You have to get together, set a time to practice, and stick to it. Take it seriously. Be there on time and be there ready to practice, and remember that you can't opt because you "just don't feel like playing today." If you were on tour and didn't "feel like" playing you can't just cancel the date. If you want the band to really work, you should be practicing once a week AT LEAST. You can start off maybe going every other week while you figure out your schedules, but the biggest thing is getting a few practices under your belt. I'd say that after five to six practices, the idea starts to solidify with most people and they start to take the band more seriously, but that's a much bigger hurtle than it seems (and than it should be).
Go over songs over and over. Don't ever make the mistake that you've practiced it enough. Instead of taking that smoke break, play through one of the songs again. Take it apart and go through it line by line, picking out the hard parts so it all comes together. Make tapes of your practices and make sure everybody has copies of when they leave. They can go home and listen to it and learn from mistakes they hear. I think this is probably the most effective way to tighten a band up, because it allows to hear how things sound when you're not "in the moment", you're just an outsider listening in.
Next time, songwriting!