Generally your melody will use the tones that define the chord notes. For instance, if you are using a C major chord, your melody will use C, E, or G, with passing and approaching notes.
Composing a Melody Example
Another approach when choosing melodic notes is to observe the overall trend of the chord progression. For instance, if you choose a common C major, F major, G major progression, and play each chord on its tonic, then the overall trend will be ascending. To create an interesting melody over this trend, you will want contrast. To create a melodic contrast to the harmonic ascension, you could have the melody line on an overall descending slope, such as C, A, G. As shown in the picture.
|Compose a Melody from a Chord Progression|
In this example the melody has an overall downward motion, with a few upsetting passing and approach notes. This downward melody contrasts with the base chord ascension, which creates tension. To counter act the tension, I created a uniform rhythm, allowing for sense of overall form during the short melodic phrase.
You can also try to find a common note within your chord progression and base your melody off of that common note.
"For example, if you have a chord progression of C major, F major, A minor, then each of these notes have the common note of C. C would be the central note of the melodic phrase."