Early versions of Blackberry devices came with an assigned eight character identification number which could not be changed. More recent versions come with the ability to change the password as well as modify default password handling. It may be that an administrator has pre-set a password for you that should be changed.
To set/reset the password:
On the Home screen, click Options > Security Options > General Settings. Note: For some versions, open Settings first. Note: If you are running BlackBerry Device Software 4.0 Service Pack 2 or earlier, click Options > Security.
Next to password, make sure Enabled is chosen,
a. Click the trackball (may need to click to left of trackball first), and then click Save on the menu that appears.
b. In the New Password field, type a password that exceeds four characters in length. Then Verify New Password.
a. Click the trackball (may need to click to left of trackball first), displaying the menu and choose Change Password. Note: for some versions, this may be Password Keeper.
b. Enter current password, enter new password and verify. Note: press the ALT button for some versions.
Click the trackball to save the settings.