There are a lot of roles that can be taken on, depending on your skill set and available time. One of the best resources I have found for information like this is reaching out to the district commissioner for more information about what kind of help the district needs. If you’ve already selected a troop, you can reach out to the SM or CC for similar information.
In the unit, there are ASMs and committee members, with the former generally being direct-contact leaders working with the youth and the latter mostly conducting behind-the-scenes support activity. Some units have specific adult roles which advise particular youth roles (e.g. ASMs assigned to advise patrol leaders or an MC assigned to coordinate with the youth quartermaster on gear maintenance and replacement).
At the district level, you could volunteer for a committee/commissioner role (e.g. unit commissioner, member of a district committee or subcommittee). At the council level, you could volunteer as a merit badge counselor. Once you have more experience, you might volunteer as a course instructor for adult leader training courses.
The key thing to keep in mind for those of us who cross over from adult leadership in a pack to adult leadership in a troop, crew or ship is that adult leadership roles are very different in the pack than in any of the other programs. In the pack, the adults are responsible for day-to-day planning and execution of the program. In a troop/crew/ship, it is the youth (scouts/venturers) who are responsible for planning and executing the program, generally with advice from the unit leader (SM/crew adviser/skipper), whose role is primarily advisory (similar to a driving instructor, who has a brake pedal, but no steering wheel). The youth do the driving for the most part, and we’re there to offer advice and, if necessary, pump the brakes when something goes sideways.
If you’ve never served in a troop as an adult leader, the fastest way to get a sense of the intended roles is to take the online scoutmaster or troop committee training. The hardest thing for most of us who served at the pack level to do as we transition to a troop or other role is to resist the urge to grab the reins from the scouts whenever things aren’t going the way we would do it, particularly when it looks like nothing is getting done, or at least not “efficiently”. That’s part of the process, and leaves more of your attention available for youth mentoring and watching for imminent “crashes” that might need intervention.