Hello! Have you looked at the new developer intro on the wiki: https://wiki.blender.org/wiki/Developer_Intro/Overview? Some links from that page that might be interesting are https://wiki.blender.org/wiki/Source/File_Structure and https://www.blender.org/bf/codelayout.jpg
An easy place to start is the blender/source/blender/editors/ directory where the editors and code for the interaction modes are found. Most of the source code that might be interesting to change is found there. Of course, if you are interested in other areas of Blender but cannot find the source files, just ask.
This topic also has some good links and discussion: What do I do now?