Colleges know that each school has its own system for calculating a GPA. Some schools use the 4.0 scale while others use percentages and some others utilize letters. This can make the process for applying to schools difficult because if your school uses a percentage system, you may not be able to effectively translate it to the traditional 4.0 scale that most universities analyze.
Most universities require a 3.0 GPA as a baseline for acceptance. This is just a general flooring and can be modified on an individual basis. Colleges may look at how difficult the coursework at your school was or other admission requirements to bend the GPA requirement. A 3.0 is considered a B average.
If you intend to attend one of the more selective universities, such as the Ivy League schools, the GPA requirements are going to be higher. Consider the University of Pennsylvania, which only accepts 18 percent of applicants. The average GPA for students accepted is 3.86. Johns Hopkins accepts 27 percent and the average GPA for students is 3.68. You will need to have a GPA similar to or higher than these to compete with the applicant pool fighting for spots.
Your GPA is impacted by your high school's courses. You may get a larger scale for honors courses, which push your GPA up over the 4.0 mark or may give you a higher GPA than if you were to take the easiest course available.