To be honest, giving you the full picture would take a long long time.
ATC is a very complex subject as rules and procedures change between different airports, different stages of flight, day and night, etc etc.
Some would say start reading the CAP493 as that is the main guiding document for ATC in the UK. Personally I don't think it's a good place to start since it would probably be as interesting as watching paint dry.
Have you seen this film?
https://www.youtube.com/watch?v=V1hCY1lHlkU
It tries to explain the basic principles in very very simple terms.
Iif you're interested in a career, why not try and arrange a study visit? You'd probably learn a lot and it could give you an indication to if it's a career you want to pursue or not.