First thing: talk with both the management or whoever non-technical you need to report to, and explain the situation and negotiate timeframe. Then talk to the t

First thing: talk with both the management or whoever non-technical you need to report to, and explain the situation and negotiate timeframe. Then talk to the techies too and kindly tell that things need to change immediately. After you've got a working time window for getting things right, prepare a workflow that should take half the time you've discussed, as it will probably take twice the time than anticipated. (if you've negotiated on 3 months of fixing the mess, assume you have only 1.5 months or even 1 month and prepare 1 month's worth of work) Then I think the very first thing should be moving to Git (or other SVN), setup development/staging environment and using CI/CD. After making 100% sure the environments are separated, start writing tests. Perhaps not hundreds or thousands at this stage, but ones that catch up the critical/big failures at least. After it start moving to a dependency manager and resolving multiple-version conflicts in the process. Then find the most repeated parts of the code and start refactoring them. As you have more time you can start organizing code more and more. It sucks but it's not something that can't be fixed. Also finally, given the work environment before you came, it might be a good idea to block pushes to the master/production branch and only accept it through PRs with all the tests requiring to pass, to prevent breaking anything in production.