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1. Choose Apple menu > System Preferences.
    
2. Click Security & Privacy.
3. On the General tab, check "Require password ... after sleep or screen saver begins." 
4. Set the time period drop down to your desired amount of time.

   1. Choose a time that's not long enough for an unauthorized person to use your computer, but not so short that if you stop working at your computer for a moment, the screen saver locks it.

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To require a password after your computer has gone to sleep, follow the directions above for locking a screen saver on a Mac.

Tips from CNET: http://www.cnet.com/how-to/7-ways-to-lock-your-macbook/

To lock your Mac: 

1. Choose Apple menu > System Preferences. 
2. Click Desktop & Screen Saver.
3. Click Screen Saver.
4. Click Hot Corners... 
5. Set one of your corners to "Put Display to Sleep".
6. Click OK.

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