Caution: if you lose your password, you will not be able to recover the document.  There is no password recovery feature in Word.

1) Open the document you want to protect.

2) Click the File tab.

3) Click Info.

4) Click the Protect Document icon in the main section of the screen

5) Select Encrypt with Password from the drop-down menu that appears.

6) Enter a password when prompted.

7) Reenter the same password when prompted.

8) Don't forget to save the document after adding the password.  As with any other change, it will be lost if not saved.

9) Next time you open the document, you will be prompted for the password.