GMATs are hard, just like the SAT's.
Scoring is similar but somewhat different as it works on an 800 point scale instead of 1600 for SATs.
Good grad schools will require several years of work experience before allowing you in irregardless of your scores, others wont care...but you will get more out of a "B" program if your associates are in the workforce as you can measure and learn from them as well.
Good Scores are above 600.
Like the previous posts, take the practice exams first.
Although you can re-take the GMATs for than once, the score get averaged when applying to school
So, if you bomb on the first one and hit 500, then retake and hit 700, your school of choice will only recognize 600
Also, some schools run formulas, such as College GPA x 200 plus GMAT score to rank applicants.
Please double check as each university has its own formulas, but this is what I recall way back when.