Hello!
I found the answer to this in a LinkedIn article, actually: https://www.linkedin.com/pulse/3-ways-locate-microsoft-team-id-christopher-barber-/
One of the ways (and the easiest, IMO) is to get it from the team's Team link. So in the lefthand pane, click the ... button next to the team (not one of the channels, but the team) and click "Get link to team".
Copy this link and paste it into Word or WordPad or anywhere really. The team id is in this URL. Find where it says groupId= and then a string of letters and numbers. That string is the team id. It ends before &tenantId.
Good luck!