Ignoring older Macs that used to ship with DVD media and a physical disc with iLife, there is a redemption process where you associate an Apple ID with the bundled iLife apps.
Additionally, both new and refurbished Mac purchased from Apple or an Authorized Reseller will have iPhoto, iMovie and GarageBand pre-installed, with no need to download anything other than updates to those pre-installed apps.
Since you are not seeing those Apps for redemption / re-download in the Mac App Store, let's speculate on what might have happened. If your Mac was purchased used, it's possible that the seller wiped the Mac and reinstalled OS X. This is common practice to ensure a clean system for the new user and to remove any of the seller's personal data. However, iLife is a separate install from OS X, so if the seller neglected to re-install iLife, that's why you're not seeing it.
If you did indeed purchase it used, your best recourse is to bring the issue up with the seller. If it was new or refurbished, contact Apple and they should give you a code to download iLife. They might also be able to help to tell you whether the serial number of the Mac you now own has already redeemed those apps or help you to redeem the license for those apps if they actually transferred to you with the purchase of the Mac.
You can start here for a link to Apple Support in whatever country you live and/or whatever country the Mac was purchased.