application to be a reviewer
The best way I've discovered to become a reviewer for a journal is to submit a paper there. In my experience this is almost always followed a few days later by a request to review a relevant article.

Edited to add: I did my first reviews while a graduate student because my PhD supervisor was the editor of a journal. He would send me things occasionally because he considered it part of my training. So, another way to become a reviewer is to know an editor.