The practice test predicts your AFQT which is based on two math sections and two English sections. Your AFQT is what makes you eligible to join and, depending on your score, may entitle you to cash bonuses and such. Scoring over a 50 is like getting an A. If you're a good reader and have a good understanding of Algebra you should score much higher. The other sections determine individual "line scores" which indicate your aptitude in different areas. The test in general should be very easy for a well rounded and intelligent person. If you aren't good in a particular area of the test (not English or math), don't worry too much. Just means you may not qualify for some jobs that you might not be very good at anyway.
Oh, and if you go into this process with just one military occupation in mind you're selling yourself short. No matter what you think you know about the military, you don't know ****. Do yourself a service and look at all options available to you. The guidance counselor's are just another layer of recruiters. They have goals to fill themselves, which is going to be the needs of the service. There is often much more available than what they initially show you. Having said that, they don't withhold stuff from you. If a particular job comes with a bonus and you are eligible for it, you're going to get it.
Just make sure you check your paperwork carefully. Some of my duties of the last few years have been helping soldiers with receiving their bonuses and such. I have seen a couple of examples where soldiers thought they had bonuses and then were not entitled to them. One, a soldier's contract stated he got a bonus but didn't specify the amount or cite the reference. Reviewing bonus messages, I could not find one that covered the time period he came in. Another, a soldier was promised a bonus when he signed into the Delayed Entry Program. Problem is when you report back to ship out, you redo some of that paperwork. A subsequent and superseding form is completed where no bonus was authorized. Another case I saw was a young lady who was guaranteed the student loan repayment program but the Army wouldn't pay. A couple minutes of research showed me that her type of student loan was clearly ineligible for the program. I have to hope that these are all cases of human error but that may be a bit optimistic.
"I would rather claim to be an uneducated man than be mal-educated and claim to be otherwise." - Wookie 03:16