If you already are familiar with one programming language (and programming in general) you should then learn: C++ and AngelScript. If you don’t know a programming language properly, yet, then you should focus first on learning to program in something like Python or JavaScript before moving to learning C++.
There are some documents describing some aspects of the thrive code here: Thrive/doc at master · Revolutionary-Games/Thrive · GitHub (there are compiling instructions here)
Other than that the only thing you can do is to jump into the thrive code and see how it works: GitHub - Revolutionary-Games/Thrive: The main repository for the development of the evolution game Thrive.
I posted some advice here as well: