The earlier advice from others re
writing tests and re-factoring is sound.
You are, however, most unlikely to be given enough
time to do it all, so you must choose wisely which
code to clean up first.
How to choose? i) write tests for all recent (and new)
bugs; ii) focus on modules you consider to be most
vital and highest risk; iii) Go through the user manual and write a test (and refactor where appropriate) for each example given there (i.e. focus on client view of the system).
Perhaps more important is to ensure that all new
code is developed test-first and with a solid test suite.
I've been (and am still going) through something similar as mentioned in What is the best way to add tests to existing code?. As expected,
and despite earlier assurances, I did not get anywhere
near the time and resources I would have liked.
Bottom line: this sort of code cleanup, while strategically
sound in the longer term,
does not bring in immediate revenue.
Update: You might pick up some good ideas
from the book
by Peter Scott. Ditto from the node starting to write automated tests.