A) you should always have a backup (preferably 2) before doing any upgrade.
B) OS X upgrades do not delete user data (but things can always go wrong, which is why you should have backups)
C) Mavericks is no longer available for new downloads.
If you have previously downloaded Mavericks, then it will be available in your App Store -> Purchases. If you never downloaded Mavericks, then you can ONLY download El Capitan for free, or you can buy a redemption code for Lion 10.7 or Mountain Lion 10.8 from the on-line Apple.com store.
If you must have Mavericks, then the other option is to go to an Apple Store and see if they will install it for you (make a Genius Bar appointment first).