There are standard guidelines for choosing passwords so that they're hard for other people to guess. Some systems even check your password and reject any that don't meet the minimal criteria. For instance, it is often said that you should have at least six characters in the password. Furthermore, you should mix uppercase and lowercase characters or include characters other than letters and digits.

To change your password, just enter the passwd command again. It prompts you for your old password (to make sure you're you) and then lets you change it.

4. Basic Unix Commands and Concepts		4.3. Virtual Consoles