You backup your data via Time Machine or whatever backup routine you have.
Then you password protect your account and create another admin account for them to test, though they have their own ways of testing and do not need access to your account and are quite too busy to look into your files or make a copy of it.
If you are really concerned, use FileVault or a password protected disk image.
LOL, Apple is the wealthiest company on the planet and has hundreds of billions to lose in lawsuits because an employee steals information, I SERIOUSLY doubt you have anything to worry about, Apple has a lot more to worry about with a rogue employee financially than I'm (fairly sure) you have.
Backup your data as instructed and take the machine in.