Usually, an average of 3.5-4.0 is expected, which means an A or A average for admission to top universities. However, you may be able to get acceptance into a less selective school with a grade point average as low as a 2, 0, or C. The high school grade point average is around 3.0, or a B average. This is also the minimum requirement for many college scholarships, although it is generally 3.5 or higher is preferred.
Finally, if you are a university student looking to transfer universities or apply for a postgraduate degree, you should know that everything here is more or less true for you. You can read more about transfer admissions here. Just like in high school, a good college average is generally 3.7 or higher, and ideally higher in the core classes. Graduate schools, in particular, tend to weigh GPAs more than scores.
If possible, consider taking a dual enrollment course through a local community college or an online virtual course.