This textbook-like-thing is divided up into sections. This section is called "Getting ready." It has several lessons in it. You can see a list at the bottom of the page. Each lesson is one webpage.
Go through the lessons from start to finish, just like a regular book. Along the way, you'll get tasks to do. Do them in the order they appear.
This content may release your inner geek.
You may experience "That's so cool!" several times per lesson.
If symptoms persist, take more lessons.
Seriously, this is fun stuff if you like to know how software works.