Unlocking everything in Rocket League is a requirement for a number of trophies, and the easily attainable platinum in Rocket League. However, you don’t have to do anything in particular to unlock the majority of items.
Unlocking all the cars in the base version of Rocket League is very simple. All but one of the cars will either be unlocked from the start or will unlock through playing naturally. The only car that is not unlocked through normal play is the Sweet Tooth vehicle which is unlocked by winning a match with each of the basic cars in the PS4 version of the game, or the Warthog in the Xbox One version. Win enough matches with enough of the cars you have unlocked and every one will be available.
It may take a little while, but unlocking cars is as simple as that! Enjoy!