Rocket League items can be obtained in a variety of ways. The simplest option is to simply play the game. Each Rocket League season has its own set of challenges. Players are rewarded with item orbs in some of the challenges. These orbs can be opened to get totally random items for your vehicle.
You will also compete in Psyonix’s competitive tournaments. The more tournament credits you get, the more you compete in a tournament. These credits can be traded for items.