This page is an introduction on how to start helping Debtags.
There are several checks that are run on the unreviewed tag database to try and detect issues in the tagging: each has its own report page, and they are good as lists of packages that need work.
You can also search packages to see if there are some that don't fit, or some that you think are missing, and review their tags.
Another good starting point is the Debtags vocabulary, which will eventually lead you to specialised tagging pages.
If you are a package maintainer, you may want to start from one of the maintainer views.