How can we break a password of a computer?
There are a number of ways that you can go about trying to break the password on a computer, depending on the type of password and the type of system you are trying to get into.
One example is a "hash" type attack or program that will string together all kinds of random words or phrases and numbers in order to try and decipher a particular password. Poorly chosen passwords and weakly encrypted password protected systems are particularly vulnerable to this type of attack.
One can also simply try to guess someone's password, particularly if you have things to guide you including knowledge of that person, their habits, etc. that might give you a clue as to what the password would be. There are also dictionary attacks, brute force attacks, etc., many of them detailed in the site below.