I am a freshman in UC Riverside right now and plan to transfer to other UCs. UCLA and UCSB will be my first choice. My major is statistics. So I want to know what I should do or what I can do to give me an advantage when I am applying. For example, should I get a part time job during summer? And what classes should I take during my second year?
go to assist.org to see what classes are required for your major at both schools. The most important thing to do is keep a very high GPA. Next you should get involved in extracurriculars, hopefully related to your major/future job. Get a part-time job, volunteer, get involved in a club, try to be treasurer of a larger club like an honor society, the list is endless. Good luck! 
Research assistantships, jobs, volunteer work (long term), social/professional organizations (clubs, honor society), student association council member, riverside county intern in the finance department… What would you like to be doing? That’s the important question that you need to answer, then you have to go do it. There are a lot of opportunities offered by public institutions such as the city council or your college. Get into something you can enjoy or at least can gain a lot of valuable experience in that is long term. Check out School-On-Wheels if you like the idea of tutoring disadvantaged youths. UC’s are research oriented, so you if can land an RA position at a lab for a quarter or more, that would help a lot I think. Tutor in stats! Or languages. Various ways you can pursue such a position, and that is very in demand. If you have the time, start up a small business obtaining free stuff from CL and renovating it for re-sell. You really have to put yourself out there and show the UC’s you have worked your ass off. That’s really the best determinant. No specific cocktail of EC’s will guarantee admission, only the demonstration of hard work, will power, and tenacity can give you a chance.
I got in everywhere I applied with a lukewarm GPA and my only EC being a relevant internship. I would try to find one.