How to get started on an existing large project [duplicate]

This question already has an answer here:

• How do you dive into large code bases? 31 answers

I have recently started a new job. My workplace uses an opensource project and I am expected to do enhancements on top of that. The problem is, I am the only developer in house. Other developers who used to work were contractors and they contributed to this open source project before they started contracting for my current company.

Unfortunately there aren't that many documentation around these enhancements and how (and where) they were coded internally. The open source project has more than 2000 Java classes. I am struggling to find any online contribution guide for this open source project. I cannot find any overview of the source code either. Also surprisingly this project doesn't have any unit tests. If I ever had to work on this large project, it either had getting started guide or someone pairing up with me and going through the code.

So my question is how do I get started on this large project?