To get productive with D3.js quickly, invest your time in coercing your data to match that of the examples. As opposed to coercing the example code to work with your data format.
Coercing your data requires use of tools you’re already familiar with (Excel, SQL, Shell Script whatever). Staying in your comfort zone here will help (quickly) get something pretty into your DOM, which you can then hack on.
While this effort is educational, it can be frustrating to get something working (and subsequently demotivating). When you’re starting out, quick and easy wins help enormously. Much motivation can come from these early dopamine responses (Ooh bar chart! Woot! Woot!).
Again, focus your time on getting your data to match the examples rather than changing the examples to handle your data.
Hopefully this will help open up D3 for you more quickly than it did for me.