You must go to the particular Group, and on the particular Group's main page you will see a Join button. 
Once you have joined a certain group, you can post in it and see all the replies to any threads there. If you want to 'unjoin' a group, you just click the same button again, it will say "leave group" if you are a group member.
Let me know if that works for you!   :)

You can start a new thread/discussion in an existing forum, but you can't start a new forum. There can be many separate discussions/threads in any forum. Hope that clarifies?

Hummm.  I clicked, "Groups", but do not see the, "Join" button...

Yes, I want to start a new forum. As a devout Luddite these thing bewilder me. A potato generally has more navigation skills than I.  

Hmmm, well you responded to my last post ok.

Do you want to create a new forum thread?  

I'm wondering if you are looking in one of the GROUPS here- each Group has its own forum too. But you must JOIN a group before you can post in it or create any threads in it.  Go to the group's home page and look for the JOIN button. Once joined you can read other people's replies there and also post yourself in the group.

Related question.  How does one post a question or respond to another's question?

Chifmunk, if you created the forum thread, you will automatically be already subscribed to it.

If you want to be subscribe/follow a thread that someone else created, just click the 'target'/Follow button to turn it yellow (active).  See my attached screenshot below to see the button in this thread, for example.  When the Follow button is yellow, you are actively following a thread.

To adjust what kinds of actions or items you want to be Notified about, go to your own profile page (click on your name link anywhere) and go to the Notifications TAB to select your preferences about getting site notifications.

follow button.jpg  •  236KB

How to subscribe/follow a forum thread...

how do I do that?  I want to get notified when someone make a new post in a thread I'm interested in.  Thanks!