How can I prevent iTunes from starting automatically on iMac?

Apps start automatically via the Users and Groups pane in System Preferences (or its equivalent in Settings if your OS is more recent). If iTunes is in the list of apps to start at login, just remove it.

Or did you mean that iTunes launches when you double click a particular type of file, e.g. video? If that's the case, do a Get Info on one file (Cmd-I) and where it says 'Open with..', choose VLC from the list, BUT MAKE SURE YOU ALSO CLICK 'Use this application to open all documents like this one'.

I don't understand what you mean when you say "every time I try to copy/move an .mp3 in another file" - have you another way of saying it?

I'm not sure it is possible? But you CAN stop iTunes being the default app for type of file - see my comment above.