First Step: Enroll

Assuming you are starting from scratch, as I was 6 months ago, here is some general advice on how to approach this language.

Enroll yourself first into a convenient class, or if you can afford it, get a private tutor. No matter how many European languages you may have learned, none of them will help you get enough of a foothold into Chinese. Just enjoy the status of complete beginner and don't demand anything more of yourself than baby steps.

As often happens when you start to learn something new, the first few classes throw up many more questions than answers, but don't drive yourself crazy by trying to get all the answers all at once. Learning a language (if you are over the age of 12 or so) is mostly an intellectual exercise but in my opinion there is still an amount of 'feeling' involved. Give it some space and don't overwhelm it with logic. There'll be plenty of time to tie up loose ends later.