There are basically two ways to compose music. One way is by starting from the bottom or the harmonic approach.

A composer/arranger takes a few chords, a phrase to hang them on and arranges the harmony in some kind of pattern. An example of this is the "loop" you often hear in contemporary music. A loop is simply a harmonic background over which a melody (or not) is played.

The second way to compose music is by starting with the melody. Composers may or may not have some idea of the finished idea (I prefer not to) but the melodic idea is fitted into some kind of phrase. The most common phrase used is the 8-bar phrase.

I find that starting with the melody to be the easier approach. Why? Because melody is easier to move forward then harmony. Sure, you can block out a few chords and arrange them to create a loop, but this becomes static over time. Melody is much easier to go forward with.

By using the principles of repetition and contrast, we can create a simple ABA form in no time at all. Then we can go back and harmonize each section.

I used to favor the harmonic approach at first. It was very easy to simply jot down chord changes on an 8-bar phrase, create some kind of arrangement, and improvise a melody on top. There is nothing wrong with this approach at all. But I soon found myself learning towards the melody first. Not because I think it's better, but simply because it's the method I like right now.

Either way, it's a good idea to compose music using one approach or the other. If you try to harmonize a melody while you're creating it, it will slow you down and may stop the creative flow.