Your band could just be you, but you'll want to split fuel costs with someone when you start touring, right? Typically, for a rock band, you will need at least one guitarist, one bassist, one keyboardist/pianist, and a drummer -- the lead singer can either play an instrument or not. Of course, this all depends on what type of band you plan to be, and what kind of music you will play. Just play what you think sounds or feels right.
The internet is beginning to offer several places to find band mates such as Band-mix and Whosdoing. If you don't have mates that are excited to jump on board, use these resources.
Facebook works for just about everything, too.
Put up ads in cafes, music shops, and even in your car window if you're feeling gutsy. Where does your type hang out? Go there. Open mic nights? Yes. Pubs or clubs? Check.
Don't use just one; use as many as you can so your chances are better.
It helps if these musicians have some musical education. At the very least, someone needs to in order to provide a voice of reason that others may not be able to provide. It is not always important to choose the "best" players. In many cases, bands of musicians who get along, are easy going, and willing to learn to play together will sound better than bands comprised of very good musicians with big egos.