You will need to earn the highest GPA that you can in high school if you want to be accepted to Cornell. According to the school's admissions statistics for the entering class of 2023, 83.4% were ranked in the top 10% of their graduating classes in high school. While Cornell does not publish the GPAs of its entering classes, the fact that the vast majority of students were in the top 10% of their classes in high school illustrates that you will need a top GPA if you want to be admitted.
You should work to earn an A in nearly all of the classes that you take in high school. If your school does not weight the GPAs of students, this means that you should strive to achieve a 3.9 to 4.0. If your school does use a weighted GPA system in which more credit is given for AP or IB courses, you should try to earn higher than a 4.0 GPA during high school. It is best to earn top grades in difficult classes. It will not help your application if you earn a 4.0 GPA in high school by taking easier courses.
If you enroll in a class in high school that you are struggling to understand, reach out for help as soon as possible. An academic subject-matter tutor might help you to salvage your grade and to get a higher score. However, you will need to be willing to put in the work that will be required of you. In and of itself, hiring an academic tutor will not guarantee the results that you want without work from you.