Scratch is a website that lets you write games and animations. It is designed to make the basics of computer programming easy, but you can use it for all kinds of things.
There are three bits to this basic tutorial:
1) About the website: it is worth looking at this, even if you know Scratch already, so you know what we call bits of it.
2) Writing a program: the basics of dragging code around.
3) Some useful types of blocks: a description of some of the basic code blocks.
That should be enough to get you up and running, but if you want to play around with the website, you could work through the Scratch tutorial.
Otherwise head back to the start page.