Scratch

Getting Started:
View tutorials "Scratch intro", "About Scratch Interviews", and Example: Image effects" here:
http://info.scratch.mit.edu/Support/Videos
Download Scratch from scratch.mit.edu. It's free and multi-platform.
Play with a few sample projects (File -> Open -> Examples). Try "remixing"/changing them.
Create a Scratch account (on the Scratch web site). This will allow you to download projects from other people's accounts and to upload projects to your account.
Try downloading a project
Upload a project you've remixed on to your site.