* Country: Grading systems differ significantly worldwide. A "B" in one country might be equivalent to an "A-" in another.
* University: Each university sets its own admission requirements. Some are more competitive than others.
* Specific program: Even within a university, different mechanical engineering programs may have slightly different admission standards.
Generally speaking, you'll need strong grades in:
* Mathematics: Calculus (including multivariable calculus), differential equations, linear algebra are crucial. High school math grades are important for university admission, and strong performance in university-level math courses is essential for success in the program.
* Physics: Classical mechanics, thermodynamics, and electricity & magnetism are fundamental to mechanical engineering. Again, strong high school physics is important for admission, and you'll need to excel in college-level physics courses.
* Science: Chemistry is often a requirement, and sometimes other sciences like biology may be beneficial depending on the specialization.
Beyond specific subjects, a strong overall GPA is necessary. Most competitive programs require a GPA of at least 3.0 (out of 4.0) or its equivalent, but many successful applicants have much higher GPAs.
In short: Aim for the highest grades possible in math, physics, and science throughout your high school and university education. Check the specific admission requirements of the mechanical engineering programs you are interested in applying to. Those requirements will give you the most accurate answer to your question.