You need to be able to:

• Write a basic html page
• Add javascript to the page
• Add a 'canvas' element to the page
• Start writing javascript to draw on the canvas.

As noted in the canvas link above, you'll need a browser which supports canvas (e.g. firefox). You can just open the .html file in the browser and you should see your code running. If you put your HTML and javascript on a web host somewhere, other people can see it too.

Have fun :-)