This achievement will accumulate over time. After every match you play (win or lose) you will receive an item drop for your vehicle. These items range from new vehicles, flags, trails, toppers, etc.
This will come naturally as you play online, Season, or Exhibition. I highly suggest you spend time enjoying what Rocket League has to offer as it's an extremely unique and amazingly fun game. If you want to get your items quickly though, use the method below.
To speed the process of unlocking these items up you can use Exhibition.
Load up an Exhibition game with these settings:
Arena/Map: Any
Team Size: 1v1
Bot Difficulty: No Bots
Mutator Settings: Max Goal = 1
Now you can pick any team you want (it will only be you) and simply make a goal which will end the match immediately. This method is very fast and can unlock an item every minute or two.
This will also unlock these achievements:
Here's a visual reference as to how easy and fast it is to create and finish an Exhibition map for this achievement: