Hi viiam,
To raise your scores from a 420 to a 680 will take a great deal of effort. Thankfully, it sounds like you've given yourself plenty of time to study for this next attempt. With this current score, you're likely going to have to work on everything and it's important to have a structure to your studies. Considering all of these factors, you'll likely need to invest in a GMAT Course of some type (either Guided Self-Study or Instructor-Led). Most GMAT Companies offer some type of free materials (practice problems, Trial Accounts, videos, etc.) that you can use to 'test out' a product before you buy it. We have a variety of those resources at out site (
www.empowergmat.com). I suggest that you take advantage of all of them then choose the one that best matches your personality, timeline and budget.
I have a few questions about your Official GMAT:
1) What were your Quant and Verbal Scaled Scores?
2) Did you have to rush to finish any of the sections (and guess on a bunch of questions)? Did you leave any questions unanswered?
Thankfully, the GMAT is a predictable, standardized Exam, so you CAN train to score at a higher level.
If you have any additional question, then just let me know.
GMAT assassins aren't born, they're made,
Rich