Play songs you already sing well and start trying with those...

Sing with someone else playing lead and you play just backup to try singing along...

Avoid tricky-rhythm pieces till you get the hang of this, such as dotted notes, since you would sing the dots but strum the straight time.

I dunno why but I think waltzes were easiest for me at first.

You have to really hear the whole song in your head, and play and sing along with THAT.

~Susan