How to learn d3js?

If you want to learn d3.js then you need to have the basic knowledge of HTML, CSS and Java-script. And you also need fair understanding of data visualization principles. But you do not need to be an expert in HTML, CSS and JavaScript, you need to have core concepts in HTML, CSS and Java-scrip.

I don't have any knowledge in d3js but I found a great resource to start with. It is an ebook for download or you can read it online with interactive content. This is Scott Murray's Interactive Data Visualization for the web that can be found on this link: Interactive Data Visualization for the Web

That is a good source. I also agree that you need to have familiarity with the basics of HTML, CSS, and Javascript for a start. You can also get some good input from DashingD3Js and other online courses out there.

What is D3.js?

D3.js is a data-driven documents library (thus the three Ds in D3.js) written in JavaScript for creating Data Visualizations that can be rendered in a modern web browser. Simple, isn't it?

How do I start?

To learn D3.js and most importantly to become productive with it, you need to be patient since it requires mastery of a range of different techniques.

So to help you in your journey toward great Data Viz, you need to take small steps and stick to it long enough to start seeing results.

Start with Mike Bostock website (creator of D3.js) and go through his many introductory posts.

You can also benefit from the official website D3.js - Data-Driven Documents and go through some code to start getting a sense of the philosophy behind this library. The more examples you code, the better you become. Event in the word of D3.js, practice makes perfect.

What to do after these initial steps?

Although D3.js is a small library, it works hand in hand with various modern web technology stacks. Among those you have:
JavaScript, as it is the language in which the library is written
SVG, which is the technology used to create scalable graphical elements.
HTML and CSS, which are the building blocks of the modern internet.