It really depends on the class. For example, in my calculus class, a D on the final dropped your grade 6 percent. But, in my statistics class, the final meant NOTHING. It was just 1/200 part of the overall grade.
Finals can be a pain in the butt, especially if you slacked the major part of the semester. To study...review, review, review, do practice problems (math and science, esp.), know what the final's going to be inside out. Do not let yourself be caught off guard.