Need Help With A Mod (or at least, how to start...)

Hi! I'm new to the modding community and I was interested in doing a modding project as a Capstone for university. My idea is to mod the University expansion pack by adding a few new majors and some new classes, but I've got NO clue where to even begin! I've downloaded S4S and extracted all the files to my modding folder as per a tutorial I found, but as for the tuning files to edit... I'm a bit lost. I understand I'll need to start adding new "tags" indicative of the major name and attributes within an XML file, but I'm at loss for how, if that makes sense. Please let me know if I need to elaborate. By no means do I want anyone to do the whole thing for me, but I just need a little nudge in the right direction. Thank you!