Such break-ins compromise the whole system, so all users must bear the resposibility of choosing good passwords. A password should be at least six characters long, preferably seven or eight. Anything beyond eight characters will be ignored.
Any typeable characters are acceptable.
DO NOT share your password or write it down anywhere accessible. System Administrators can give you a new temporary password if you forget it. You must change this immediately, using the UNIX passwd program. Change your password periodically. Users who have accounts at other sites should use a unique password for each account, in order to contain the damage done if one of the passwords is compromised.