I Found the hardest part to making it sound good was relating the lyrics to chord changes, while still staying on beat. I still fight with this problem, but a lot of songs that i know well i can sing wiht them.
A difficult thing to overcome at first is trying to make it sound exactly the same. First just work on getting it all out together, then If you still want it to sound exactly the same you will have to work with a metronome to really nail the timing, but i have never bothered,
Just try singing to an easily strummed song with a basic up down motion, You will have to listen to the record and compare the chord changes to the lyrics, or simply find a tab that some kind soul has already listed the chords with the lyrics.
Start simple and work your way up, I still have a lot of trouble with Picking and singing, but practice makes perfect :D