There are many prep companies that can teach you all about the GMAT™, ranging from the more traditional names--e.g., Kaplan or Princeton Review--to online prep courses with video tutorials--e.g., Magoosh or Manhattan Prep. There are a lot of such businesses that, in all honesty, have in-the-know tutors who can help someone prepare for the test from the ground up, so to speak. You can check dedicated online forums such as GMAT Club to see what users have said about their own prep courses. No one institute seems best, but several have high reviews from a lot of happy students. And of course, there are always private tutors as well, some of whom have worked for prep companies in the past.
Good luck finding the best fit for your needs.