The basics for FMP 8 and FMP 13 are the same when it comes to password protecting a file. The following just limits access by setting a "full access" password so that only you can access the file. The first step is optional, but makes it a bit easier to log in when you first open the file:
Go to Privileges and note the the user name specified for your FileMaker Pro install. If you set up an account with exactly the same account name, FileMaker will enter the user name as the account name and you'll only need to enter the password that you specify.
Make a back up copy of your file so that you can be sure to open a current copy even if you make a mistake with the following steps and accidentally lock yourself out of your own file.
Open Manage | Security.
enter the user name as your account name. (Or you can enter any unique account name that you want.)
enter a password (This is case sensitive so consider how you will capitalize parts of your password.)
Select [Full access] as your privilege set
Unless you have changed settings in File Options your file is set to automatically enter an account name of "admin" and no password when the file opens. To get FileMaker to pop up the log in dialog, disable the admin account by clearing that check box here in manage security.
and Enter your new account name and password when asked to do so.
When you close and re-open this file or use the re-login script step, you should now see a dialog that automatically opens where you must enter your new account name and password to open the file.
Since you have multiple devices opening this file, you may or may not have the same user name specified on each. On machines where a different user name is specified, you'll need to clear the user name that appears as the default account name and enter both account name and password to open the file.
That worked great, thanks for all your help!