The structure of in-person and online meetings varies from group to group, but most encourage you to share your story with the group (within your comfort level), support others in the group, and reflect on ways to improve your life going forward.
A large component of support groups is spending time talking with other people who are in the same situation as you. This "peer support" time together has been shown to be incredibly beneficial and often results in higher substance use abstinence rates and feeling more satisfied with substance use treatments.