There are about 5 thousand ways to go about learning to sing. Here are a few important points that I've found are almost universally true.

1) The more you sing, the quicker you will improve. Just like others have said, sing in your shower, car, practice room. It's just like playing your guitar. The more you play the better you know your instrument. You start to notice little things about how it should feel when you reach certain parts of your range.

2) Even though it seems tedious and unnecessary, vocal warm ups make all the difference in accessing the difficult parts of your range. I have a CD with a few basic warm ups (played on the piano) that I listen to almost every day in my car on the way to work. In the last 6 months I've gone from being comfortable hitting a G to being comfortable hitting a Bb and I attribute much of that to this warm up CD. 2 other warm up tips I use all the time are to warm up in the shower (because of the steam) and to eat something before you sing. Something about the contracting of the muscles in your throat helps your voice warm up quicker.

3) Listen to a recording of your voice. Much like guitar, besides pitch, the most important thing that can change the sound of your voice is tone. It's really hard to hear the tone of your voice because of the way it resonates in your head. Listening to recordings helps with this very much.

4) Never underestimate the many benefits to be had from taking proper vocal instruction. I wouldn't take lessons from anyone that doesn't have a masters in vocal pedagogy unless they have uniformly good reviews from his/her students. Bad vocal advice can hurt your voice in the long run. It's very important to protect your instrument.

This is a link for the CD I use to warm up. It's crazy expensive so I just downloaded it from a torrent. I know this isn't legal or moral, but charging that much for a product is absolutely ridiculous.

http://www.singingsuccess.com/