That is a thing that I used to struggle with- singing and playing at the same time. I am more of a vocalist than a guitarist to be honest. I was a professional vocalist for a while before I got serious into playing the guitar. It came one day when my lead guitarist got his hand injured from an accident. I only sang at that time- no instruments, but we had a gig that night and I had no choice but to replace him (i was the only one who knows the solo to the songs). Since no one in the band can carry a tune, I had to learn how to play while singing at the same time. Some people say that it is easy to sing while playing rhythm but I noticed that singing while playing rhythm can be as hard when you sing and play the bass or lead guitar. This is because singing takes a lot of energy when you do it PROPERLY. Try this excercise for istance. Try singing with an accompaniment (ie. karaoke) and give it your best. Remember to sing the song PROPERLY!- sing with your diaphragm, make sure you give emphasis to the words (diction), make effective use of dynamics etc. and then try singing the same song while playing a guitar. You'll notice that the song is not quite the same. It is not as powerful and you experience a decrease in volume. Sometimes, it can even throw off your guitar playing.

It took me awhile to get over this problem. Like what the others have said, practice is the key. But how to practice is another question. You can practice as much as you like but if you don't do it right, you won't get too far. For me,
studying the song and breaking them into small parts are the most effective. Learn where the notes that he sings lands on notes of the guitar and make it your guide.
You can sing with the original cd so you can familiarize youself in what point in the riff does the artist sing and take cues from. This is very difficult but again, think of it as a listening excercise. When you've become familiar with it, try to sing and play the instrument with the cd and FOCUS!!! I find that it is better to start with slow songs that use short licks. When you get better, you can try one with longer riffs. When you've mastered this, you can progress to faster songs. It takes a good while but if you're patient enough, you'll be able to pull it off.