Scott Murrays website is a good first start http://alignedleft.com/tutorials/d3/.
Once done with that, I might recommend you take a look at this website http://techslides.com/over-1000-d3-js-examples-and-demos/, it lists 1000 examples of d3. You can usually copy the javascript data and code and play around with it locally on your own computer.
You can also try to take one of these plots and see if you can reconstruct it.
The reason why I found this to be useful is that you need to be able to recognize the errors that d3 throws at you. It's the moments that you have no idea why there is an error that are troublesome. By having an example that you are reconstructing you guarantee yourself of a copy of code that works such you might be able to more easily correct yourself.
