How to be a competitive transfer applicant?

Hey CC. So seniors finally got their decisions and are now in the process of accepting which college/university to go to. I decided to go to my safety for the first two years to complete the required course load and then transfer during my remaining two years to do my actual major. I’ll be applying to around 12 universities after 2 years and some will be really hard to get into. I would not have made it in as of right now as incoming freshman applicant pool at prestigious universities would be highly competitive. However, since none of this (high school GPA, SAT, SAT II’s, etc.) matters for a junior transfer, I want to know what will. What makes one highly competitive? I didn’t have too high of an SAT score (according to CC lol) because I was oblivious to the college application process and how hard it would be. I started to study late and all. I want to know everything before I go to college, however. What do I need to do to get into schools such as UC Berkeley, UIUC, Carnegie Mellon, etc. for Computer Science?? All I know is you need a high GPA. What else? And how do I start doing it the moment I step into college?