There are a number of different Wikipedia Backlog categories that help track the existing backlog of referencing activities. They fall into two major groups, Lacking References and Ensuring Accuracy through existing references:
At the moment, there are over 351,819 articles with "Citation needed" statements. They are some of the easiest referencing issues to solve, because you are frequently looking for a targeted and singular reference. You can browse the whole list of these articles at Category:All articles with unsourced statements.
With 351,819 statements that need WP:Verification, sometimes it's hard to choose which article to work on. The tool Citation Hunt makes that easier by suggesting random articles, which you can sort by topical category membership.