When you enter tin, you see the groups that you have subscribed. You can read messages posted to these groups by using cursor keys and enter. You exit from articles, threads, groups and finally the program by pressing 'q'. You can see other groups that you did not subscribed by 'yank in/out' command. Then you can highlight the group by moving the cursor and 'subscribe' to that group. Also you can 'unsubscribe' a group that you do not want to read. When you enter a group, you can 'list thread' of an article, that is the list of replies to an article. When you are in a group, you can 'post' an article to the group. Similarly when reading an article or a thread, you can 'post a followup' that is a reply to a message. You can also 'save' an article or a thread that you wish to keep in your own directory (in regular the 'save'd article is put under "News" directory under your home).
How to give these 'commands' and other commands is listed in tin program by pressing 'h' (help command) in group selection menu, group menu, thread menu and while reading an article. Also pressing 'H' key toggles mini help menu for quick referencing simple commands.
Do not forget that we have a "test" group if you just want to try posting something to "tin".