Apple - 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.

I enable the Guest Account in System Preferences, then go to the login screen when someone else wants to use the computer. That way they get a totally clean user with none of my personal settings, and anything they do is wiped from the system when they log out. It's super handy.

If you want really 99.999% security, the only way is to encrypt your data. (Because in the worst case, it's possible to open your Mac, take off the HDD, and read it in another computer).

To enable encryption, you can use FileVault:

System Preferences > Security > FileVault

See: Use FileVault to encrypt the startup disk on your Mac

Ps: Honestly, I never tried it. Maybe someone can post more about it.