I am currently a Junior in high school with a 2.5 GPA. I really want to go to a UC or CSU. What should i do?

It is unfortunate that you will not be able to apply to any UC campuses because you need a minimum 3.0 GPA for UC. But I heard about cases in which applicants with GPA slightly less than 3.0 were admitted to UC Merced. Otherwise, you would only be admitted under very rare circumstances (with outstanding talent and achievements in sports and arts, you know what I mean).
But you can still apply to less competitive CSU, such as CSULA or SFSU, given that your SAT score is high enough (check with the index: http://www.csumentor.edu/planning/high_school/cal_residents.asp)
If you have set your eyes on better UCs, you can enroll in any community college, work really hard and get transferred two years later.

Get your GPA to at least a 3.0 and EXCEL on the SAT, because with a 2.5 GPA, you will not get admitted to any UC or CSU campus. MOST of them (except maybe the lowest ranked campuses such as CSU Bakersfield and CSU Dominguez Hills) require a 3.0 now to get in.