This is how I learned. I started on a song with 3/4 timing, and moderately difficult to me back then. "Wondering Aloud" by Jethro Tull. (If you start out with a harder song, you'll find it easier to just add on easier songs.) Then I practiced the song till I had it down perfect, and added on the vocals. It took a lot of repetition. But after I figured out how to do, its cake now. Its hard for me to imagine not to be able to sing and play at the same time. It isn't possible. Good luck.