mdh has nutshelled for it you - there is no one way to progress or create when composing as there are so many paths you can travel.
Most pop music is based within one of several scale/mode choices:
Major: Major scale/Lydian mode/Mixolydian mode
Minor: Natural minor scale/Harmonic (and all of its modes) minor scale/Melodic (and all of its modes) minor scale/Dorian mode/Phrygian mode
Chord progressions can be chosen before or after scale/mode choice: i.e. both provide a pointer to the tonality/modality in use.
Instrument choices are an important of composing as they form the basis for expressing your ideas.
Structural ideas are also important as they allocate/relegate different instruments to different roles. There are two levels of structural ideas:
- Level 1
- Melody: the main melodic ideas singer, solos, etc
- Counter-Melody: the response to the call of the melody
- Harmony: chords
- Rhythm: percussion and bass (bass also draws on harmony)
- Level 2
- Formal structures
- Ternary: ABA | AABA | ABAA | ABBA | etc a lot of RnB is based in this type of structure
- Binary: AB | AAB | ABB | etc sometimes called Verse Chorus form, the B section can be broken down into other formal structures as well
- Rondo: ABACABA and variants (also called ritornello or returning form) Fur Elise is an example of this one
- Song Forms: Intro Verse Chorus Verse Chorus Solo Bridge Chorus Verse Chorus Chorus Outro is one of many examples
- In each of the above no indication has been given about numbers of bars for a section (AB or the various song form sections) - these can be from 4 to 8 to 16 for an individual section. The AABA and variants is usually called 32 bar song form, i.e. 8 bars per section letter. AB is usually called 16 bar song form for the same reasons. However, there is nothing stopping you from taking these structural components and making the smaller or larger
When analysing harmonic progressions (chord progressions) we usually use the letters found in the level formal structures to represent different parts of the chord progression as they relate to the overall structure.
Think of the level 2 structural ideas as the scaffold to hang your level 1 ideas on (your chord progressions and melodic ideas).
The rest is simply choosing a path and experimenting as you follow it......
If uncertain about some terms in this post, use the search here at fp to find deeper explanations