As a general rule of thumb, anything over an 85 on the ASVAB will qualify you for nearly any position in the armed forces. But there are slight breakdowns within each score. For example, in order to qualify for Surveillance and Communications (SC) in the Army, Word Knowledge, Paragraph Comprehension, Arithmetic Reasoning, Auto & Shop and Mechanical Comprehension all require high marks. Though scoring an 85 or above would guarantee you scored in a high enough percentile to qualify for SC.

First, the ASVAB measures your accumulated knowledge in different areas. So if you are currently in school, stay focused and study hard. Your overall knowledge gained in school will most likely be reflected in your ASVAB score. Second, though accumulated knowledge is important, you may be able to raise your score by preparing specifically for the ASVAB, especially in areas where you are weak. Various study guides are available for this purpose (listed left).
You can take the test as a junior or senior in high school and use the score to enlist, provided that you are at least 17 years old and took the test no earlier than 2 years before you begin enlistment processing. If you are at least 17, you can take the test at a Military Processing Station (MEP) or a satellite Military Entrance Test (MET) location.
One ASVAB sub-measure, the Armed Forces Qualification Test (AFQT)--which is just a separate analysis of the same tests that focuses only on four of the subtests--is also population-referenced, but its score from 1-99 is a percentile score. This means that if you score a 73, you scored as well as or higher than 73% of test-takers in the national sample.