How can protect all privacy from someone that needs to use my Mac?
Just make sure your admin account has a good password that isn't written down anywhere (or add some numbers you will remember to your usual password.)

From there, you can make a guest account for them - as long as it isn't an admin user - they can't grant themselves access to your files.

If anything is truly sensitive - don't let them near the mac. It's easy to boot any mac from an install disk and access your files since my operating system won't care what your password was. Your guests would only need time, a little knowledge and curiosity (or worse) to have access to anything that isn't otherwise encrypted on your Mac. You can (should?) make encrypted disk images to store any sensitive information that you wish to protect from someone that would access your mac against your wishes.

See this article for an explanation. Even if you store your password in your keychain (less secure) - a technician can't see the contents of the folder unless they crack your password (or your keychain/log in password). It will just be scrambled bits.