Answer:
The most common form of 'Hacking' users accounts on websites, or computers is simply to acquire that users password. It is reasonably simple and unglamorous, very unlike anything seen in the movies.
The other way is can be achieved is to exploit venerabilities in the system to bypass user passwords or to retrive the password once access to the users data is achieved. This is not realistic for most. Trying to hack major companies websites is close to impossible. They may even be able to track your IP adress, and find out who you are, and prosecute you.