You have to memorize the chords and lyrics.I could probably learn guitar but how to sing too?
I asked this same question to some industry pros about 8 years ago. I couldnt get my hands to do what they had to do while concentrating on the lyrics and melody. I'll tell you exactly what they told me: It just happens. The answer wasnt satisfactory to me at the time, it just sounded like he was saying "I'm just better than you," but it's true. Eventually you get comfortable enough with the guitar that it becomes almost an involuntary function of your body. To practice this, though, is rather annoying when you first start.
I would pick a song with a simple chord structure and one that has an easy and unchanging strumming pattern. Sublime was usually good for that. Boss DJ or Saw Red come to mind. Once you have a song that you are very familiar with that fits that criteria, attempt it first at a reduced tempo. Soon the contrast between the rhythm of the lyrics and guitar become a pattern that is comfortable to you and you will be able to do it in your sleep.
At a certain point in learning the guitar this technique will become moot. You will eventually be able to sing to the most complicated songs with no problem.