depends on what kind of music you play and what your goals are. Going on a full-blown tour takesa huuuuge ammount of effort and usually a well-established network of contacts, and a decent fanbase.
To book a show, the "official" way of doing it is to send the venue a presskit, whether you give it to them and talk to them in person (the best way to do it) email them, etc etc. Another common way to get on a bill to to have a band that is headlining offer to you open for them.
If you are a smaller band or an acoustic deal, or solo artist, why not book your own show? I booked my own shows in my hometown/area in cafe-like palces, and as long as you have a fanbase and you promote it enough, they turn out to be a lot of fun.
I got a show on Haight St. In SF over the summer at The Red Vic. It was a lot of fun, and a very nice turnout. I did that through messaging the dude that sets up the shows on myspace. So be sure to consider the internet.