Just in case, I’ve included stuff about major pre reqs, but since your question seems more EC oriented, feel free to skip:
assist.org will give you the classes you need to complete the lower division requirements at different UCs. Make sure you read it carefully, since some classes are REQUIRED and others RECOMMENDED. Also, it may list a ton of classes, but only say you need 1 or 2 of the ones listed.
IGETC is the equivalent of General Education at the UC level. It is not required, but it can be helpful to clear your GE requirements and allow you to start taking upper division courses right away.
TAG uses major requirements, a certain amount of units, and GPA. Will you have 30 units COMPLETE (not including in progress) and a 3.4 by September?
Here are some useful links:
assist: http://web2.assist.org/web-assist/prompt.do?ia=UCI&ay=15-16
IGETC: http://admission.universityofcalifornia.edu/glossary/igetc.html
http://admission.universityofcalifornia.edu/transfer/general-education-igetc/igetc/
TAG: http://admission.universityofcalifornia.edu/counselors/files/tag-matrix.pdf
UCSB TAG: https://admissions.sa.ucsb.edu/applying/transfer/tag
Also, a good thing to keep in mind when planning courses is Major Prep > IGETC
Re: ECs, any type of volunteering is good, related clubs, really anything to show that you’re not just studying 24/7