Let's begin with the basics. The Scratch website is obviously a great place to get started with Scratch, but here are direct links to the most useful sections for beginners. Of course, it's not the only place to look when you're learning to Scratch, so here are some others too.
ScratchEd for Teachers
Scratch can be used to learn about math concepts, scientific principles, language arts skills, and all sorts of general creative thinking and problem solving activities. The ScratchEd website is a great place to look when you're ready to start integrating Scratch into your regular curriculum.