ACCESS CATEGORY IN REVIT FOR SCHEDULES

Hi Everybody, I've been using Revit for a while but never had the chance to learn Revit API until now. I am currently on my Lesson 6 on "My First Plug-in Training" and hopefully will learn how to write code and create plug-ins. Is it possible to write plug-ins to access categories that is not included for schedule? also if you can give me tips on what's next to do after finishing my Training, I am considered as a total beginner in programming. any tips will be greatly appreciated. Thank you :)