To prepare for GMAT you have two main sources: one is the OG (Original Guide) and then all the others.
Also to prepare for the test, first you need to build your basics and then practice the questions.
To build your basics you can use any good resource. You can even use your school math and grammar books. Even youtube have lot of good content for this, especially related to GMAT.
Now coming to the real practice, OG is the best resource. Do not be in a hurry to complete all the questions. Go slow, assess your mistakes and go back to build on the basics based on the mistakes.
Other than OG, Manhattan GMAT is a good supplement.