How to get the latest version of GarageBand on my MacBook Air?

may somebody please help me figure out how to install the latest version of Garageband for my MacBook Air, early 2014 Model #A1466? I believe it isnt allowing me to download it as it has been saying, "cant be installed on "untitled disk" because OS X version 12.3 or later is required?! Well then,..... .... how the heck am i supposed to get the latest OS X version of 12.3 or later on my MacBook Air?

[Re-Titled by Moderator]