By default, your Mac logs in to your user account whenever it starts or restarts. But that’s risky. If someone else gets control of the system, they’ll have access to all of your data. That’s why it’s smart to require a password at login. Go to System Preferences and choose Accounts. Click the lock in the lower-left corner of the window, enter an administrator’s credentials, and click OK. Select Login Options (at the bottom of the account list) and choose Off from the Automatic Login menu.
Choose the List Of Users option (the default) or the Name And Password option to specify what should appear in the login window. Make sure your password is secure, passwords become exponentially harder to crack with each character you add, so longer passwords are much better than shorter ones.