Hi AllisonW,
While it may be tempting to put off your studies until July, you should still put some time and effort in during the month of June. Many Test Takers need 3 months (or more) to hit their respective peak scores. An August 21st Test Date means that you have almost 3 months to study, which should give you plenty of time to become properly versed in all the various aspects of the GMAT.
Studying for the GMAT is essentially a part-time job; plan to put in 10-15 hours per week (and possibly more) during your studies. Sometimes the work will be "easy" or "hard", but since the GMAT is a standardized, predictable exam, you can train to crush it.
GMAT assassins aren't born, they're made,
Rich