Once you've signed up for a race, you'll need to create a training schedule. There are tons of resources out there to help you get started, from books to Web sites to magazines, so there's no need to reinvent the wheel. One thing to remember is that it's important to consult your doctor before beginning any kind of training regimen, especially if you have any pre-existing health problems or aren't in top shape.
When you've established your training schedule, stick with it! The race will be here before you know it.