To book a tour, you need recorded material. If you haven't yet recorded any songs, do so immediately and make sure they are professionally done.
Once you've recorded material, set up a webpage where you can feature your music. You'll need a band/artist bio, photos and an email address where people can contact you and sign up for your mailing list. You will also need a main contact phone number.
Set up pages on social networking sites and start culling for fans.
Put together a mailing list to expand your fan base, which then expands your options for bookings. You can implement a mailing list option on your website, so fans can sign up there, or they can sign up in person at your shows. Whenever you contact people on social sites, ask them if you can add their email address to your mailing list for upcoming shows.
Decide on a budget before you contact anybody about booking your act. You need to know how much money you are working with. How much you can afford to spend, how long that translates into life on the road and an estimate of CD/merchandise (if you have other merchandise) sales all dictate where and how long you go on tour. If you have $5000 to work with, and don't mind living in a van, you can do a several-month tour, but you have to realize you won't be staying at the Four Seasons and you may need to survive on granola bars for a few weeks.
Once you have a budget, figure out a route. If you have a strong fan base in the southwest, start researching venues in the New Mexico/Arizona/Utah/Nevada/Southern California area. However, keep in mind, many places in Los Angeles will not book you unless they know you can bring a draw (an audience) of 50 people or more. If you can't feasibly pull that many people in to your show, don't bother contacting that person (yet). The worst thing you can do is book yourself somewhere where you need a draw, and then not be able to deliver the number of people. The bookers remember, and it's a small industry. People talk.
Research venues. Know their capacity (how many people they can accommodate), their location (within city limits or outside town), their style (what types of music they book), their booker names and contact information, their booking policy and names of any other artists you know who've played there. Most of this information is on each venue's website. If it isn't, don't be afraid to call to find out who the booker is and how to contact them.
Pool resources. There are tons of networking sites out there, with artists in all areas of the world. Identify artists at your level who play your type of music. Contact those artists and ask them about their experiences with the venues they've performed in.
Once you've put together all these factors, start contacting the venues. Always be polite and ask if they'd like you to send samples. Give them a brief, unique introduction about yourself and why you'd be a good fit for their venue. Also let them know when you are planning on being in their area. Try to give them three specific dates, but let them know you can work with them if those particular dates don't work in their schedule. Be sure to check the venue's schedule first before you give them dates, though if they already have someone booked on the dates you've mentioned, they will know you didn't bother to check this out beforehand, and will think you're wasting their time. DO NOT attach anything to the email unless they ask you to send something. If you know other bands who've played at their venue that are a similar style, mention that. Then leave them alone for about two weeks before following up. In some cases, the booking policy says no phone calls. Be respectful of their policies and show you've paid attention by not calling. They will contact you if they are interested and think you would be a good fit.
Always keep detailed notes in a database on every bit of information you come across. Keep track of who you've contacted and when, what you've sent them, what you need to send them, and who and what you need to follow up with and when.
Be flexible but persistent, be organized and be efficient. Always offer to send the bookers whatever they need. Make it easy for them to book you.
Once you've gotten enough of a name for yourself, start looking for possible endorsement deals to help fund your next tour.