Purchased MacBook Pro a day after Yosemite release but got Mavericks instead
Simple!

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.