Hi there -
On your profile, you'll see a list of communities that you belong to. Next to each one is an "x" and "leave". If you click either of those, you will leave that community.
You can "watch" any thread without joining the entire community. You should see "Watch this discussion" under each thread. You can click on that and it will show any replies in your Watch List. You can stop watching at any time, as well.
Let us know if you need further assistance.