When you're looking to start a band for the first time, it may be hard to know exactly what the initial steps are. The good news is that all you really need to know in the beginning is what style of music you want to play and have already written a tune or two. After that, it's all about creating and maintaining relationships.
To begin, you will need to have already written some songs that you want to play with other people. Sometimes you may just have a riff or two on which you would like to collaborate with another musician so it can develop to its fullest potential. This, too, is a great starting point. When you have done those things, call or email your other musical friends. They may play guitar, bass, drums, or piano. It doesn't really matter. What is important is that you begin collaborating.
On the other hand, let's say you have these riffs that would be perfect backed by a rolling bass line and some enthusiastically-pounded crash cymbals, but you don't know any drummers or bass players. What's more, you're the only guitar player you know. If this is the case, the first thing you will want to do is go see live bands in your area whose styles are similar to your own. Get to know some people there. These are your contacts! Talk to members of the band after the show. Tell them you like what they're doing (if it's true) and you're looking for some people with whom you can work on similar material. People who are already in bands love creating music with other people, and there is a good chance they'll be willing to sit down with you and collaborate on some tunes. If their current schedules are too busy to start working on other projects, it isn't unlikely they will tell a friend that there is someone looking to put some songs together. Either way, you have nothing to lose by getting making contacts with other musicians whose styles and musical interests are similar to your own. Sure, it can be a little intimidating to approach people with whom you want to create music. In the end, however, the great music you create together will be well worth any anxiety you experienced in the beginning.
An additional approach to starting a band is to check your local classifieds, especially alternative weekly papers. A lot of publications even feature a section especially for musicians looking to get together with other musicians, which provides great opportunities to connect with people specializing in various styles and who bring many different musical experiences to the collaborative table. This is also a great way to begin speaking to musicians who may have more experience than you do--people from whom you can learn. The only caveat is that you will want to speak at great length with any people whose names you glean from the classifieds before meeting them in person. Creating music should be an enjoyable experiment, and you should always feel comfortable around those with whom you are working.