I am happy to help.
For high school: Take the IT/Computing classes provided. This will, at the very least, provide you with more exposure and should help close some of the gap you may face when learning the materials for the certification exams. Do not expect to get the real meat of your education in computing from high school - you can get that through post-secondary education.
After obtaining the certification: The CompTIA A+ certification is a great foundation certification, and should be considered the launching ground to start a career. Education-wise you should consider other certifications such as Network+ and Server+ from CompTIA, certifications
from Microsoft, and so forth. Pacing is really up to you, and you can obtain a certification in a few weeks or few months (up to you). In the IT industry, "advancement" is commonly limited only by knowledge (ie. certifications) and experience (ie. work experience).
Certifications do not necessarily show "expertise". They simply show you possess "X" level of knowledge in something. The term "expert" is commonly thrown around all over the place, but in general a real expert is someone who possesses not only the knowledge but the experience in a given field. Which, if you think about it, makes perfect sense. Once you start looking into the certifications, what they teach and what they ask for, you will get an idea as to what it covers.
As for which exams to take, take a look at the following graphical table
from the CompTIA A+ certification page which equates what you need to pass to gain certification:
Addendum: I should note the A+ certification was changed a little over a year ago, so "technically" there is more than one "flavour" of A+. The older version, such as those obtained by older farts like myself, used to be just one certification obtained through two exam components (Hardware and OS), however with the new version this is no longer the case.