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 426,464 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 426,464 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.
The Barnstar of Reliability is awarded to editors whose contributions improve the reliability of Wikipedia articles. To award this barnstar, click the WikiLove heart icon at the top of a user talk page (on a desktop or laptop computer), choose "Make your own", and use Barnstar of Reliability.png as the image filename.