Studying for STPM is not easy, requires lot of hard work. I suggest you start with the basic steps, just like in SPM level:
1. set up a timetable for studying and follow it.
2. talk to your school teachers on the subjects you are taking, regardless if the subject is easy or hard. make sure you meet your teachers from time to time. do proper arrangement and ask them when can meet them to ask about certain topic etc.
3. build study groups with classmates. inquire where they tuition, in case you need extra coaching.
4. surf the net for pass year papers, tutorials etc.
5. be focused in school and class.
6. be strict at home, and don’t simply follow family every where for holidays, weddings, dinners, death etc.
7. but that doesn’t mean study 24 hours, you also have time for hobby or sports or entertainment. importance is balance.