There is no real folder locking that works, IMO. Your simplest bet is to do this:
Open Applications->Utilities->Disk Utility.
Select File->New...->Blank Disk Image.
Name it, set the size to what you'll need, set the encryption to AES-128, and set the format to sparse disk image.
It'll then create the image and ask you for a password. Be sure to uncheck the option to store the password in your keychain.
Now you're done.
Double-click the image to open it after providing the password, use it as you would a folder, and then eject it to close it. It is completely safe and can only be seen if you leave it open.
If you don't like the ".sparseimage" after the file name, select it, select File->Get Info, and tell it to hide the extension.