Championship Points are explained here. Basically, you get more points the higher you place in a tournament, and the players with the highest total points get invites to Worlds (Nationals can be entered by anyone, but winning a regional gets you byes for it). If you don't have enough, you can also get in by entering the Last Chance Qualifier which takes place the day before the main event. Your other question is really broad, and I'm not quite sure what kind of information you're looking for. You don't need to worry about embarrassing yourself, I've seen plenty of casual players there so as long as you know the basic rules of the game and pay attention to the announcers you'll do fine.