how much do high school grades matter in usc, and how high should you score on the sat to be accepted

High school grades and coursework matter significantly to all colleges, and definitely to selective ones like USC. Generally, schools will have a cutoff or threshold below which they will accept very few to no students, and you'd need to be above that threshold to move to the later rounds of the admissions process, where things like ECs and essays are more important.

That said, a 3.7 should be fine for getting you over the academic threshold for USC. SATs work similarly, and typically you want to be at least above the 50th percentile (considering that the majority of students below that point have some kind of hook—athletes, legacies, etc.), and ideally at or above the 75th percentile. You can usually find that info on any school by googling; for USC, their average is a 1440 and 75th percentile is a 1530, so ideally you'd want to score a 1500+.