That all depends on you.
Can you self study and put the time to identify your weaknesses, using online resources such as GMATClub and commit yourself to prep?
Or do you need help developing a study plan and some instructor time to get over certain obstacles?
I personally needed a prep course to get over certain obstacles. If I had dedicated more time here, that might have not been necessary, but I don't regret it. The folks at
MGMAT were great and the class was well worth the price I paid.
Read through some of the more well written debriefs in the share your gmat experience forum. Start off with a practice test (the free one from the prep companies:
MGMAT, Kaplan, Veritas,Knewton, etc or use a GMATPrep test) and figure out what your weakest section is. Work from your weakest up. If you don't really know where to start, the generic content, such as Kaplen GMAT Premier, is an excellent starting point. Take practice tests through your prep to gauge progress and figure out what your roadblocks are. Keep a good
error log.