If it is a trigonometric function at the junior high school level, it is basically the study of right triangles, and some special triangles can be solved after expansion. I think it's best to learn or understand in advance: the similarity of triangles.

If it is a trigonometric function in high school, you need to be familiar with it on the basis of trigonometric function in junior high school: plane rectangular coordinate system, arc unit of angle (these two are used to define trigonometric function) and congruence of triangle (used to derive some inductive formulas). This knowledge point is actually junior high school geometry, which you can learn by the way before learning similar triangles), algebraic operation (which is also the content of junior high school mathematics teaching), and function (which is briefly mentioned in junior high school and systematically defined in senior high school, so as to fully understand it as far as possible).

I'm not a professional teacher either. The above is for reference only. My suggestion may be, look directly at the trigonometric function part you want to learn, and supplement your knowledge if you can't. Look at the preparations above.