Hey there,
First off, congrats on ur choice. As far as your question. The ASVAB scores(more specifically your GT score off of it) is what the military uses to see who is suitable for which jobs. In most cases, those who score high on their GT score have more options, and those who score very low are usually limited in choices(most commenly things like Combat Arms and 88M-Drivers).
First off, go to your local recruiter station, and take the practice version of the test they have. It doesnt take very long, and it will give you a good idea of what to expect on the real thing. If you do poorly on the practice test, go to a book store, and pick up a ASVAB study guide, work with it for a few weeks, go back and take the practice test again. Once you feel comfortable, head in to MEPS and take the test. If you have the option of taking it on the computer over the paper version of the test i recommend using the computer. There are several timed sections that you will get a higher score on with not having to stop to take time to fillin bubbles with a NO. 2 Pencil.
As far as intelligence goes, i was originally a 11B(infantry) and switched over to 97E(HUMINT). So i can tell you, that in the military, the intelligence field is open. Most simply spend the minimum required time in order to be eligible to gain a job doing intelligence work on the outside as a civilian contractor. So you should be good their.
Just a nother tip. Dont let the Career counslers tell you that "There is no slots for "XXXX" job, these are your choices." While they might be filled up for that day, you can always come back tomorow or the next day, or next week and gain which job you want. Remember, the military is hard enough with out getting stuck in a job you dont like. But over all it is a great career, and i am willing to bet you will be happy with it. Just make sure you have a good recruiter working with you, WHO IS BEING HONEST with you. If u suspect he is just trying to get you in, and not do what is best for you. Request another recruiter.
hope this helps
Lance - currently in afghanistan :D