There are many great resources on the web - use them!
Cole has spent a lifetime turning data into pictures. After a spell in banking, Google and other corporates she has honed her skills and developed herself as trainer helping organisations present their data visualisations more effectively. Follow her on twitter @storywithdata.
This is the home of d3.js - and possibly the most important resource and starting point. Use it learn what D3 is, what it can do, and see some great examples.
Dimple.js is an object-oriented API for business analytics driven by D3. It aims to ease the learning curve and reduce the amount of code required to make business type charts.
blo.ocks.org is a Mike Bostock creation to view code examples on Github Gist. His own blocks comprise hundreds of examples he has created.
Find hundreds of examples to see what D3 can do, and use as a starting point for your own creations.
Buy it from O'Reilly as an eBook, or at the time of writing you can read it for free online. This is a valuable resource for learning D3 from one of the experts in the business. Scott's concerns are more than technical as he takes a design approach to data visualisation as well.
Udacity provides online "nano-degrees" developed by its heavyweight industry partners. The courses within the degrees may be taken for free, and this one is part of the Data Analyst nanodegree. I found this to be very well set up and constructed and is a great starting point if you are new to data visualisation as well as D3.