By now you should be familiar with the vocabulary and definition part of the J Dictionary.
This primer has introduced most of the J Dictionary concepts, although in a simpler way that is restricted to specific situations. As mentioned before, the J Dictionary is a reference book and its descriptions are both as concise as possible, and at the same time as complete as possible, with more emphasis on complex and tricky situations for experts, than on the mundane ones for beginners.
It would be worthwhile to read the sections that preceed the individual definitions. Much of it will make sense, and some of it will either answer questions that had arisen in your mind, or just as likely raise interesting new questions. Based on the experience of seasoned J programmers, you'll find yourself reading and rereading this material, and on every reading you'll learn something new.
You are also now more than ready to attack the Introduction. You should definitely work your way through the first 20 lessons. It shouldn't be too tough and you'll learn a lot.
Especially if you work through the exercises!
The remaining lessons are more difficult and will take more effort to master.
Working your way through the Sample Topics is one of the best ways to meet some of the primitives in action. You'll be continuously challenged and constantly referring to the definitions and experimenting with the system to try to understand what is going on. In the end you'll be a much better J programmer, to say nothing of learning a fair bit of math and computer science!