Either this or the other way around. I started as a singer before I ever started playing guitar so learning lyrics and the vocal melody is a second nature. Once I have the song down vocal-wise, integrating guitar is easy, provided I don't have anytyhing too fancy to play (complex chord progressions are fine and simple leads too but I can't solo or do complicated leads while I sing, at least not that well).
Remember that if you can't sing the song correctly without playing guitar, there's no way you'll do a good job once you have a guitar in your hands. The opposite is of course true as well.
In the end, it comes down to what has been said: practice. When you get comfortable doing both well individually, combining them will require a bit of practice and you'll be good to go.