there are plenty of ways to begin with GMAT. The 11th Official Guide is a must and I also strongly recommend you the Manhattan GMAT books
. They are expensive, but they're focused more on the theory than on the tricks to solve the exercies, which is what Princenton and Kaplan
That should take some time and in the meantime you can begin with tests. A diagnostic tests is a good idea, but not all of them are reliable and you will find out that the good ones (GMATPrep and Powerprep mainly) are scarce, and I definitely would save them for the end of your preparation.
I also strongly recommend you to review your mistakes in the OG (there are some people who have an error log
, I just crossed them in the book) and do them again until you get close to 100% right.
After doing all that, you should be able to score above 650's (depending on where you are). And to get the >700, well, that's another story.