Make a recording of the chords you've worked out. Play it back and fool around on your instrument until find a possible melody line.
Think about what inspires you, and play around with that theme for your vocal melody, using everyday language for the listener.
Pay attention to songs by different artists that you admire, and take note of the format of the lyrics. Does the artist choose to rhyme the end of each line, or do the ending words bear little similarity to the other lines?
Be careful when choosing to rhyme your song. Make sure that the rhyme makes sense, and doesn't sound like you're trying to fit a rhyming word with a crowbar.
Take the song in whatever way it unfolds. Sometimes a vocal melody may come to you first, while the next time you may find a chord progression that's new and different.
Study music theory to grasp an understanding of the possibilities when writing music. Most songs feature several standard features, including times when the chords are what the listener expects, as well as chords that leave the listener wanting a resolution, such as 7th chords.
Pick out notes that are from the chord for your vocal melody most of the time, but throw in a few surprises to keep it interesting for the listener. Varying the rhythms can bring a new twist to the song.