To expand your garage (the smallest one into the small one or the small one into the big one) you need to click on "upgrade" located in the garage manager. Confirm and the garage will be upgraded at once and you will be able to buy new trucks and employ new drivers.
Each European city you explore offers an empty small garage. To buy it stop at the green € icon and confirm the purchase by paying EUR 180,000.
It will become yours immediately. Now you can buy new trucks and hire new drivers. You can even move there and make it your new headquarters.