@Student25050: Do ECs that you are genuinely interested in doing, not simply “drive-by” ECs or ones that you participate in simply to fluff up your resume (and admissions officers can tell). Admissions officers want to see you doing things that you are passionate about, not what you are mercenary about. If you can demonstrate leadership in your ECs, that is also a plus.
I don’t know about raising your GPA by any way other than keeping your nose to the grindstone, doing your homework, “picking the low-hanging fruit” in your classes, etc. For raising your standardized test scores, you might want to consider taking a test prep course (my kids did this with a live person, although there are internet test prep courses available – Khan Academy comes to mind).
FWIW, here is link to the U of M Common Data Set: http://obp.umich.edu/wp-content/uploads/pubdata/cds/cds_2016-2017_umaa.pdf . Part C7 tells you what admissions factors are considered, and how they are weighted, by U of M; Part C9 of the CDS gives the median 50% for both SAT and ACT scores, as well as the percentage of the entering first-year class falling within certain ranges of SAT and ACT scores; Part C11 gives the percentage of entering first-year students falling within a range of unweighted GPA on a 4.0 scale, and Part C12 gives the average high school GPA of first-year applicants.