As for putting a melody over your chord progressions.
I usually start by singing while playing the chords. Your ears will automatically give you a general idea of which notes are going to harmonise well and which ones wont. The gravity of the harmony itself will also drag your melody in the direction of the chords.
Scale choices.
If your progression is diatonic then you will usually get away with just using the major scale. For example, if you look at the ii-V-I in my last post. All those chords are from the key of C, so C major scale will sound nice over the entire sequence.
As for the following examples I gave. There will be many more options. Such as the melodic minor/harmonic minor/ bebop scale and more. But this would require a much deeper definition.
In fact you could use these scales over the simple ii-V-I, if your wise about it.
Songwriting is a fine art, and one should not expect to be great at it within a short period of time. Some people are natural composers but most of us need to study these things in detail to really grasp what the possiblities are. In the end though, with all the information in the world, it is up to you and what you are hearing in your mind.
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