It’s a good idea to organize your photo library BEFORE you start building your Lightroom catalog. It is important that you know how to find any image on your computer without Lightroom, (or Picasa, or iPhoto, or Microsoft Picture Manager, etc.)
Lightroom is a powerful database than can help you organize and find things at lightening speed, but it is only an application. If you've been around computers for even a short time, you know that bad things happen. Sometimes applications don't open when you need them most.