First off, I'm assuming you've already qualified (through your division or section) for some/all of your events, since going to Nationals without having qualified is kind of a pain, and I don't 100% know how it works.
Okay, assuming that, you'll probably want to enter.
Here is the entry form for the individual events, and
here is the one for the team events. (You'll need Adobe Acrobat, or something else that can handle .pdf files.) The actual schedule is
here.
As for the due date, it's coming right up--May 27th, I think. The USFA being the super-organized and on-the-ball organization that it is, get that puppy in
early. The fee is pretty steep, something on the order of $50 just to show up plus $30 per individual event entered. If you're late, though, you can take that number and triple it. Late entry = bad news.
Everything else you'll need to know should be contained in the entry forms themselves. Good luck!