Before start apply for IPTA, make sure you have purchase the UPU code from Bank Simpanan Nasional (BSN) which cost for RM15.60.
The next step is that you will need to do a little bit of research for the course you wish to apply. You must apply the course according to the CGPA pointer you get in your STPM. For example, if you get the CGPA of 3.0, it's better for you to apply courses which the pointer is close to yours CGPA. You can check the CGPA pointer for most of the courses in IPTA in this website, QuanSheng. However, the CGPA listed in this website is just for reference.
Besides that, you may want to look up for special requirement for certain courses in the UPU website.
After you have done all your research, you can apply for IPTA (for all the IPTA in Malaysia except USM)at UPU Application website.
However, if you wish to apply courses in Universiti Sains Malaysia (USM), please proceed to USM website to do your application.
All the best in your application!