So I got admitted to UCSD with my 2nd choice picked major. Is there any possible way I could do to make my first picked major be waitlisted or anything similar.
That is not how it works. You were accepted into your 2nd choice major, so if you want to get into your 1st choice major, you will have to fulfill the requirements needed to switch majors. What are your 1st and 2nd choice majors? Waitlisted applicants have no guarantee that they will be accepted into either of the majors and may end up with Undeclared.
My first choice major was computer science and my second was applied math. So just to clarify, if I go to UCSD as an applied mathematics major and want to switch my major to computer science, do I only take classes required for computer science major or do I also have to take classes for applied mathematics? Thank you for your help.
You would have to take the following classes to switch into CS. I am sure several classes would overlap with the applied Mathematics major. You can work with your advisor during orientation so you can achieve the best schedule possible in case for both majors. Your chances will dependent upon your GPA in these classes and the available spots.
Minimum of eight units of courses completed at UC San Diego for a letter grade, drawn from the following courses: CSE 8B or CSE 11, CSE 12, CSE 15L, CSE 20 (or Math 15A), CSE 21, CSE 30, and CSE 100.
Thank you so much 
Do you think I should go to UCSD hoping to change my major to computer science or should I just go to UCI or UC Davis with my first choice major as computer science?
@Futuralism Just posted my answer to almost the exact same question in another thread. Here is my answer there:
If you are sure you want to major in CS it is too risky to go to UCSD. A little bad luck or bad professors and your GPA won’t be high enough to get in.
Go for the direct admit. Both schools will give you an excellent education.