To tell you the truth from the beginning, getting to the point where you're able to read the code in existing class or style files (which do make extensive use of TeX and not only LaTeX) takes some good deal of time and effort.
Usually the easiest thing to do in order to “customize” an existing package is to first load the package in question (without modifying it), and then redefine just a few commands in order to fit your needs. The best way to learn is by doing it, so try to do something very specific (i.e. try to change the specific way something is formatted) and ask a question here if/when you get stuck.
It might also be useful to browse through other questions here, particularly those tagged
macros, in order to get an idea of what other people is doing and what does their code looks like.
Finally, in order to get some type of documentation for reading TeX code in existing packages, have a look at TeX by Topic, or one of the other suggested books in this other question.