Complete the following steps to secure your computer.
Update your operating system.
Check the UNIX or Linux vendor web sites for new security patches weekly, and immediately after any announcement of a vulnerability.
Password protect your accounts.
All active accounts should be given strong passwords.
Disable default accounts.
Some UNIX systems come "out of the box" with a number of accounts created and active that you may not need. Accounts like "printer" and "guest" are good examples. If you do need these "default" accounts, you should set strong passwords for them.
Check your password file for unexpected accounts.
Any account in group 0, or with a UID of 0, which you did not create, or are unaware of, is highly suspect and could mean that the system has been compromised.
Disable services you don't need.
Any services which are not needed for your day-to-day use should be disabled.