This will sound stupid, but start with songs that you know like the back of your hand. Happy Birthday (not the Beatles) She'll be coming 'round the mountain, favorite Christmas carol-something you can sing without any thought at all.

Strum straight-down and up, and sing. When you get the hang of it, change your strumming pattern, until you have the best version of Happy birthday ever.

In the future, you can't learn 2 things at once. You have to either have the lyrics and tune down pat, or the guitar part. Then put them together.

As with everything-it takes time. The hardest thing to do is sing the tune, not what you are playing. (although, if you sing what you play, you can sing harmony)