For a step-by-step introduction, download the Scratch 1.4 Getting Started Guide. The Scratch 1.4 Reference Guide has a thorough explanation of the Scratch interface and programming language. Scratch Cards provide brief explanations that show how to make animations and interactive projects with Scratch.
Display: 800 x 480 or larger, thousands or millions of colors
      (16-bit color or greater) 
      Operating System: Windows 2000 or later, Mac OS X 10.4 or later,
      Ubuntu Linux 9.04 or later (For other versions of Linux, see the
      Linux Installer page) 
      Disk: at least 120 megabytes of free space to install Scratch. 
      CPU and memory: Most computers have enough memory to run Scratch
      1.4, but very old computers may run Scratch slowly. 
      Sound / Video: Sound playback requires speakers (or headphones),
      and recording requires a microphone. Many laptops have speakers
      and microphones built in. Scratch 1.4 can use a USB or built-in
      camera (optional).