0-6 correct: Easy
7-14 correct: Medium
15-20 correct: Hard
The first section is weighted more strongly than the second section. To get an idea of how you can earn the score you want, take a look at the table below, which lists a myriad of different score scenarios on the GRE PowerPrep CATs.
As you can see, the total number correct is not always an accurate predictor of your score, because a heavier emphasis is put on the first section than the second.
The key to a solid GRE score is to "qualify" for the medium or hard sections on your first scored section--doing so automatically improves your score.
Of course, there is one major caveat to all of this: when you are taking the GRE, you are trying to get every question right, and you don't know which questions you are getting right and wrong. So in many ways, you are operating in the dark. However, this table should at least give you a better idea of precisely how you need to perform on the test in order to achieve your desired score.