For teaching students beyond compulsory school age, all you need to have is plenty of demonstrable experience and knowledge in your specialist area. However, pretty much all colleges are now asking that you have a teaching certificate and/or a PGCE in Post-Compulsory Education. Many will let you study for these 'on the job', as it were.
When I was at university a few years ago, we were told the minimum qualification for lecturing there was a Masters, and some of our lecturers also talked about a PGCE for teaching at university level.
For newcomers to community education, full-time work is rare. You usually have to get a foot in the door via the part-time route. Around springtime, community colleges advertise for part-time tutors. I know GCSE law courses in our local area are usually well-attended, so if you could familiarise yourself with the GCSE syllabus, you could find a couple of evenings' teaching a week for starters. If you're lucky, you might also land yourself a funded place on a teaching certificate course.
You might also think about teaching any related subjects as an alternative. That would depend on your specialism as a solicitor (if any), but many years ago, for instance, I was taught business theory by a law specialist.