The first step is to generate ideas. This can be in a number of places depending on preferences and perhaps the style.
Starting with the chords.
Chords are the building blocks of a song. Putting together the chords in a sequence that makes sense to you and to the listener. Most Pop/Rock groups use the same chord progressions.
As found here
To experiment with chord progressions you can take a familiar chord progression and add augmented/diminished or suspended chords to add interest and tension to the song. You can even rearrange the order, add/takeaway or substitute chords for other chords.
The example below shows the progression I V vi IV. The progression has been rearranged to a V vi IV I which gives the piece a different feel.