license
A mechanical fitter is basically an unlicensed mechanic. The requirements are usually an engineering (preferebly aircraft based) apprenticeship and/or maintenance experience.
You will be much better off if you have a few modules under your belt before you apply, it will show you're dedicated and eager, as i said the maths and physics are no harder than what you will be doing at college. If you get your head down you can rattle off a module every month or two.
As for experience the best way to get this is in a base maintenance environment, you will learn more about the aircraft systems and structures also you will not get a line position without experience and a license.
Some companies will help you out with training etc but these are few and far between in my experience.