Hi Naomi and Sophie,
most of Chromium is written in C++.
Learning how to program is a great plan, and I highly recommend it, as it can be lots of fun! However, C++ is not the most beginner friendly language, and Chromium's code base is so huge that it's not the most suitable project for beginners either.
You might also want to get started with some general introductory programming course. There are plenty of books as well as online tutorials; however I don't know any of them well enough to give a particular recommendation.
Have fun!
Jakob