Monitor your local NWS office site. URL is
https://www.weather.gov/ plus the three letter code; i.e.
https://www.weather.gov/box/If you look there (Boston/Norton), the first "headline" is about upcoming spotter classes. Each office will have these on a different schedule and if they are done for the year they will not be listed.
The Boston office also has a specific SKYWARN page.
https://www.weather.gov/box/skywarnIf you don't find anything like this on your office's site, look up the contact info and email/call. Ask to be alerted when the next set of classes is scheduled.
The class I went to (four years ago - up for refresher next year) was good. Lots of info on thunderstorm anatomy, how to positively discern key features from "scud", how to report, etc.