http://www.jazz2online.com/jdc/info.php

Read the "What is JDC?" section of this page, and the two sections that show the rules. That will give you the best overview. Note that you can't do anything until tomorrow, when the tournament starts.

It's up to you to arrange your own duels, 2vs2 games, etc. You can also join events, which are posted on the site regularly (but join early, because they usually fill up fast).

Edit: oh and make sure you get jj2+. The vast majority of JDC users have it installed, and it's pretty much expected these days (most events will probably require it from now on).