What you charge will depend on what you intend to teach. If you are planning on teaching students (for example) how to pass one or both of the A+ exams, then you can charge higher than if you are tutoring them in their IT classes or helping them learn an application for homework or whatever.
It also depends on your experience and the time you take to prepare for the class and whether or not you hand out guide sheets or instruction manuals etc.
I don't think location matters much, but you may have to balance what you charge with your area in order to not be seen as having high prices. Eventually word-of-mouth gets around, and people will pay what they can if you provide a sufficient service to them.
$20/hour to learn a new application with guidesheets/overviews that you provide is not unreasonable for one-on-one. For "classes" you can charge a little less, but you'll be dealing with a topic and guidesheets; they'll come to you in off hours if they need extra help.