Well, there isn't much help on the 'net (for free, at least). I was looking pretty hard about a year ago, until I finally broke down and bought an internet study package (the one advertised on the main page of this site). I haven't had the time to really go through it thoroughly, but it seems like it could work.

Basically, the only advice/help you'll ever get without going to a real vocal coach is in breathing exercises. Even a vocal coach will get you to first do breathing exercises before anything else. You need to learn to breathe in fully, into the diaphragm, then be able to control the flow of breath out of your mouth.

After that, it's basically practicing scales to improve your pitch and voice control. There's no magic pill to make you a better singer overnight; you'll need to practice every day, just like the guitar, esp. if you haven't been blessed with a nice voice to begin with.