Watch the Develop the Riff online guitar lesson by Ravi from 1-2-3 Songwriting
Start by playing your riff over and over - loop it. Explore the dominant changes that you hear in the riff, as these will start to form your chord progression even if you never actually play them as chords. If your riff is short and there are no apparent chord changes, try changing just a single note each measure, or whenever rhythmically sensible in your riff. This is the beginning of turning a riff into a larger section. Count out loud while playing so you can get a sense of how long your section should be. Start riffin' and check that off your list!