Hi,
First thing i'd recommend if you haven't done so, is to read this whole thread. Its long, but it has some very very useful info about the whole selection process.
Secondly, i would definitely recommend getting some flight training under your belt if feasible for you to do so - what better way to prove your enthusiasm and interest in aviation than actually being behind the controls of an aircraft!
Thirdly, i would start reading as much as you can about all things plane related - start off with basic principles, then progress from there. Another reason why flight training is important is that you should be studying theory along with your lessons (so the theory makes alot more sense). Start off with Basic Aeronational Knowledge type level material, then progress from there.
As for how you should show your enthusiasm, that can't be taught, it should naturally come out in the interview. If aviation really excites you, they will see it in the interview. They will see it in the way you answer your questions, facial expressions and tone of voice. But don't be fake.
Don't forget, there is more to the selection process than just the interviews, there are medicals, aptitude tests, personality tests and flight grading, so you need to make sure you do not neglect those aspects aswell.
Goodluck!
