Your GPA is the most important factor that colleges use to accept or not accept students. To get into a competitive college, it's important to get high grades and take challenging courses. This usually means AP, IB, or honors level courses.
There are two common ways of calculating GPA in the US - the "4.0" scale and the "100" scale. The former, which is more common, uses the grade of A as 4.00, A- as 3.67, B+ as 3.33, B as 3.00, B- as 2.67, and so on. Depending on the school, honors, AP, IB, and accelerated classes may be "weighted" to increase the GPA of those who take harder courses.
The latter system is less common, but easier to calculate. Instead of using letter grades, it directly takes into account the numerical value of your grade out of 100.
Your guidance counselor should be able to tell you your GPA.