generally they ask for an admin password and username so that means they can gain access to any of your files if they so wish. Here is what I do.
Firstly I create a new account so that I won't have to give out my password, or I at least change my password to something else such as "pleasefixmymac"
Then create an encrypted disk image in Disk Utility
So open up Disk Utility, can be found in your Utilities folder. Then click the new image button
Choose a size that has some headroom over and above the ammount of data you want to add to it as the volume won't be the full size described
Most important step is to make sure that the password is not remembered in the keychain which is selected by default for some unknown reason.
Then the image will mount on your desktop and you can put your files in there and then once you have done that you can unmount the disk image and they will be secure.
It is best to do both of these things.
Also one thing to note is that if you give them your admin account that they will be able to see the content of the keychain so I would suggest changing the password on that as well. So open up Keychain Access (in Utilites) then Edit - Change Password for Keychain "login". This will cause a dialog box to pop up when you sign in and try and use items from the keychain but that can be resolved once you get it back from the apple store by simply changing the password back to the same password that you use for logging in.
However it is still best to create a second account for them as there may be little things that are lying around that they can see, but it really does depend on how paranoid/need for security you are.
