How would I write a thesis statement on how being a college athlete affects a student's GPA?

A thesis statement is one sentence (or, at most, two sentences) at the beginning of your paper that tells your reader what your entire paper will be about. It usually can be found in the last line of the introduction.
The thesis statement is the focal point of your paper. If you are being asked a question, the thesis statement must clearly answer that specific question. It often makes a claim or introduces your interpretation of a topic. The rest of your paper must then present evidence to prove that your thesis statement is correct.
If you are writing a paper about how being a college athlete affects a student’s GPA, you could argue one of three positions. First, your thesis statement could be: “Being a college athlete does not affect the GPA of a student.” Conversely, you could argue that being a college athlete either negatively or positively affects a student’s GPA. “Being a college athlete negatively affects the GPA of a student” and “Being a college athlete positively affects the GPA of a student” are two thesis statements that you could use to clearly express either of these points.
Keep in mind that in order to prove any of these thesis statements, you will need sound evidence from reputable sources to support your points. Your points would need to be organized into clear paragraphs, and your essay would need to end with a conclusion. You should also include a Works Cited.
http://grammar.ccc.commnet.edu/grammar/composition/thesis.htm

https://www.bucks.edu/media/bcccmedialibrary/pdf/ThesisStatementsandIntroductionsJuly08_000.pdf