Practice matching the pitches of single notes. Start with the fourth guitar string (D). Pluck the string and hum the pitch you hear. Raise the pitch by fretting up the neck for three or four frets. Do this with the second, third and fourth guitar strings (they're closest to most people's singing range).
Strum a few chords and hum what you hear.
Begin with a very simple song such as "Mary Had a Little Lamb." Strum the chords to this song and sing along. It might seem elementary, but it's best to start simply and with something very familiar to you.
Move on to more complex songs. Try something on the radio that you like, but don't pick a song with incredibly difficult vocals. Try to choose a song that's sung by a person with a voice that you think is not too far from your own in tonal quality.
Make sure your playing is automatic. If you're playing the guitar, you need to be able to do it very fluently as you sing, or your singing will suffer because you'll be concentrating too much on your playing.