For example, try surfing to http://jsfiddle.net/rniemeyer/LkqTU/ and you'll see the example below. Adjust any of the code you like, click the Run button, and see the results instantly.
For something more graphical, try http://jsfiddle.net/zalun/FZqZN/ and move your mouse over the output window while the program is running. Also try http://jsfiddle.net/pyrliu/JPEEv/2/ for some great examples of animations and charts.
Search in Google for jsfiddle examples and you'll find loads more. Then you're ready to start work on your own masterpiece of programming.