I don't know any websites, but personally, I just started with my CD player! Seriously, man, just close the door to your room (and the windows, if you think anybody might see you), put in your favourite CD, and just rock out, singing along as you go! Concentrate on following the melody as closely as you can to begin with; when you're reasonably good at that, you can work on facial expressions :D. Eventually, try picking up your guitar and playing as you sing.

But that can only get you so far - if you want to be a really good singer, you'll probably need a couple of lessons.