You unlock new garage items after each game you finish so just finish 5 games (when I started I already had some progress towards this, maybe from the back to the future dlc) and you will unlock this achievement!
You do not even need to win the games to unlock stuff!
As far as boosting for lots of item unlocks:
- Go to Exhibition
- Arena = DFH Stadium (any arena will do though)
- Team Size = 1v1
- Bot Difficulty = No Bots
- Go into Mutator Settings and change "Max Score" to 1 Goal
- Create Match
- Choose either team
- Just hold your boost right off the start going directly at the ball, just as you are about to hit the ball brake (milliseconds before, this is just to prevent the ball hitting the crossbar instead of going in)
- Skip the highlight, hit ok or equip on the item you unlocked then hit ready to rematch the bot
Each match using this method only takes 20 seconds. obviously not a big deal completing 5 matches normally, but the "Drops in the Bucket" achievement requires 50 items, and the "Stocked" achievement requires 200.