I have been there and if you are not a newbie, you have probably been there but if you haven't been there, don't worry you will be there. There will always be a time in your life when you inherit code that was written by another developer. If the developer is smarter than you, that's a problem and if they are not, well that's a huge problem. This is a problem most developers face when they start a new job at a different company.
Recently I was given a task to maintain a project that was written by another developer using a programming language that I am not familiar with. This article has some few things that helped me get up to speed with this new project as well as explaining these ways in case you encounter the same problem.
Once you get an understanding of the programming language, take your time to read through the project code. Go through the code as much as you can if possible from line 1 to line n, file 1 to file n. Going through the code thoroughly will help you understand the way the developer structures their code, their commenting style, strength and weaknesses. This will also help you know where to edit when you want to edit any features of the project.
Experiment with the project, make a backup then break it and fix it. If you break the project you will suffer, the suffering is necessary because it will help you understand the concepts and technologies used in the project when you are trying to fix the project. This will also help you to understand several modules of the project and their connection.
NB: There are several ways that can help one to get started with a project that was written by another developer but this is what worked for me.