There are many different positions at an Apple Retail Store. They can range from Store Manager, Mac Genius, Mac Specialist, Keyholder, Inventory Specialist, etc..
The 2 most popular positions are the Mac Genius (Mac Technician), and Mac Specialist (Mac Salesman).
To become a MacGenius, you'll need to get AppleCare Certified. There's a kit you can buy from Apple (AppleCare Certification Kit). I believe its somewhere around $299. This will prepare you for the AppleCare Certification Exams. There are 3 exams that you will need to pass in order to become a full AppleCare Certified Tech. There's a Desktop exam, Laptop Exam, and Mac OS X Exam. Each exam will run you around $150 each. A MacGenius not only fixes the computers, but also answers questions at the Genius Bar about anything Mac related whether its a question about a printer that connects to your Mac, or a piece of software that installs on your Mac. So its a lot more than just knowing about the Macintosh Computer itself. They're also responsible for keeping the other store employees up to date on new Apple technology and products that are announced. They're almost like 2nd in command in an Apple Retail Store (next to the store managers of course).
A Mac Specialist are responsible for being out on the floor and answer any questions that people have while looking at the display Macs. They're basically the salesman of the Apple Store itself. In the larger stores that have theaters in them, I believe they're responsible for giving the in store training sessions that people pay for. Those stores have training sessions for different topics that people can purchase, sit down and learn right in front of them. The topics range from OS X itself, Final Cut Pro, iLife, Certain Parts of iLife (ie. Just iDVD), etc...
Keep an eye out on Apple's website for job openings so you can apply for them. If you're selected (which many applicants will be), they'll call you for a phone interview and then you'll either be let go from there, or they'll ask to do a regular sit down interview. Then you'll either be let go from there, or you'll go on for probably another interview in which they'll choose the people they want for that particular position from there.
If you do get an interview either phone or in person, make sure you bring along any questions you have. They're more than happy to answer any questions, although certain questions they won't answer in detail. (ie. How much exactly does this position pay?)
This is all from my experience as I wanted to work for Apple as well. I got 3 interviews for a Mac Genius position, but in the very end, wasn't chosen.
You can buy the AppleCare Certification Kit on Apple's Online Store:
http://store.apple.com
Alternatively, you can buy 2 Books from Amazon.com which are AppleCare training books. These are significantly cheaper than the kit and very in depth. Here are the links to those books:
Mac OS X Training Book:
http://www.amazon.com/gp/product/03...f=pd_bbs_1/002-9725252-6400006?_encoding=UTF8
Desktop/Portable Training Book:
http://www.amazon.com/gp/product/03...f=pd_bbs_3/002-9725252-6400006?_encoding=UTF8
Both versions are Apple Certified Training Books.
You can check job postings on Apple's website:
http://jobs.apple.com
BTW..You will need to be at least 18 years of age to be even considered. And you will also have to pass a background check as well.
Its extremely competitive to for all positions so you'll really need to show your passion for the Mac. Anything you can think of to say that shows you passion for the Mac. Also, they focus a lot on customer service, more so than the technical portion. You can be the smartest Mac person in that area, but if you're not a people person and aren't always willing to help others, then they'll consider someone who knows less than you because they're better with people. In other words, they won't even consider someone who sounds like they'll do a halfass job. There's too many other people they could choose from. There's enough people who want to work for Apple where if they want you and you're interested, they'll come get you.