Will Carington, works for Noodle
Because grading practices differ from school to school, it's hard to say exactly what grades you'll need in order to get into MIT. However, it's safe to say that your grades should reflect your ability to excel in a challenging academic environment.
For information about MIT admissions, take a look at the admissions section of MIT's Noodle profile, which provides a few useful pieces of data – for example, that 98% of incoming freshman at MIT graduated in the top 10% of their class. Another useful resource is, of course, the MIT admissions website.
Hope this helps!