Many of the Baronies within the Kingdoms do demos at schools and such but not so much for the 'public'. It's a historical group so the idea is to find your local group, then attend 'Council' meetings and get to know the people and what they do.
You can find out if your local group has equestrian events (some areas don't, but that doesn't mean you couldn't start one!).
Once you show an interest in participating you will have people at your beck and call working to get your persona (who you are going to be) developed and how to get involved with the group.
I have a close friend who was involved with the PA SCA for years! She was actually apprenticing to be a Knight until her health problems (which always looks funny 'cuz she's a wee little lady!). Then she switched to Seamstress. Anyways, she was going to let me tag along, but then she stop going for personal reasons. Now she lives in Georgia.
I tried talking w/someone involved in the local equestrian SCA. They weren't friendly or really willing to talk w/me.
My other qualm is I think the SCA given timeline for personas isn't as early as I would like for a persona. I think.