Back up your data to the iCloud or an external drive using TimeMachine. Here's how (link)
Disable “Find My Mac” (Required for Apple Service) Here's how (link)
Remove all covers and cases. Do not bring bags, boxes, accessories or power cords. (Unless necessary for a power issue.)
Wipe down unit with a disinfectant wipe (such as Clorox Disinfecting wipes) or 70% Isopropyl alcohol, then dry with soft dry cloth. (Wipe the external and internal areas except for the track pad.)
When dropping off your computer, include the following information (on paper, such as Post-It note): Name, your best contact phone number; and log-in credentials/passwords if issue is software or system related.
For iPhone, iPads and other devices
Follow these simple steps...
Back up your data to the iCloud or an Apple computer. Here's how (link)