Gnus Group Buffer
To see all the groups from your servers active file, you can type A A. In theory this could take a while, but I haven't been on dial-up in a long time so...
Typing L will list all the groups in your newsrc file. Typing l will list the subscribed groups in your newsrc that have unread articles. These commands really are all about your newsrc.
Select a group by pressing Enter. It will by default show the unread articles unless you have a display config item set. If there are no unread articles, it will offer to display them all.
G DEL to delete a group
F to fetch all new newsgroups. This is useful if you've just configured a maildir select method.
Group parameters
Use G c to configure topic/group paramters. You can configure anything topic/group specific by adding the variable local to the group (see local variables at the end of the configure buffer)
Which articles to display?
You can set a group parameter (G c, when over a group in the group buffer) to display certain articles upon entering a group. Look for the display config item.
Once in the group, you can limit the articles by using /.
/ u will only show unread articles. / o will show all, read and unread
Getting new articles
In the group buffer, type g. In the summary buffer type M-g
Catching up
Pressing c will catchup (mark as read). You can do it in the summary buffer, the group buffer (if you've selected all the groups you want), or if the topic you want is selected.