Watch the Create an Intro online guitar lesson by Ravi from 1-2-3 Songwriting
The same guiding principals for an intro apply here as they did everywhere else. The only difference is that since we have a riff (perhaps your musical hook), it might be best to set up the song with that main idea right from beat 1. This could be especially effective if you needed to simplify your riff later to accommodate the melody in the verse or chorus. If you introduce your song with the full riff, the ear will habitually hear it later even though you simplified it for the melody - see the example in my song. Experiment and see what sounds best to you, and check that off your list.