I know they probably have stocks to finish, but I waited precisely one day after the presentation of Yosemite (October 16) to order my new Mac hoping it will come with Yosemite preinstalled.
Bummer, it came with Mavericks.
So what do I do next?
First you open the Mac App Store. It should be the blue 'A' icon in your dock. You can sign in by going to the Quick Links section at the side and click sign in. Here you enter your Apple ID and password when prompted.
Then go to Updates at the top and OS X Yosemite should show up. Then you click Free Upgrade and it should download and install. Once it's finished installing you can open it and go through the procedures in the installer.