Help studying???

Currently I’m in high school and these are my average grades for the first two semesters of this year:

Concert Band - A+
Intro to Humanities (English) - B+
Intro to Humanities (Social Studies) - B+
Honors Biology - B-
Honors Precalculus - C+
AP Statistics - B-
Spanish 2 - A

I really need help with getting my grades up. Any tips on specific subjects? I really want to end the year with a strong GPA. I think my cumulative GPA currently is about a 3.4/3.5. I used to get all As but now I get Bs and Cs. I really need help with some tips on some specific subjects or some tactics you guys used.

Why do you have a C+ in Honors Precalculus?

Because I am terrible in math…

Why are you “terrible at math”? What do you normally do to try to circumvent that? What has happened? In order to improve your study skills, we need details.