Go to 'Open Invites' at the top of the page. You can accept a game offer there if you are eligible, or you can create your own 'open invite'. Is that what you mean?
edit- I should also mention that you can challenge someone directly as well, if you don't want to use the 'open invite' page. Just click on their username and you will see an option to send them a challenge.
Single games are what we mostly play here, you can't enter a tournament unless you become a subscriber.
All games on the site are "public games" that anyone can view if they wish (they only see the "game history" page, not the main board, and not any in-game messages or notebook entries which are strictly private).