The command to change your password on most Unix systems is passwd.

Picking a new password

• The new password should be eight characters long. You may come up with a password longer than eight characters, but the computer will only recognize (and use) the first eight.
• The password should consist of lower-case letters, upper-case letters, number, and some punctuation.
• It should NOT be a dictionary (including non-english) word or a name!
  
• Choose a password you can remember. One simple way to do this is to use the first letters of some sentence you will remember. For example "this is one really stupid password!", which becomes ti1rSpw!

Using passwd:

The following transcript shows what may (or may not) happen while changing your password with passwd on a Unix system that uses NIs. Note, your password will never echo to the screen.