From the outside of your home, open the housing menu and choose "Create Parking Spot".
You will see a green radius around your home. You cannot create a parking spot (or place furniture) outside of this green radius. Press G where you want to set your parking spot to be.
You can try out your new parking spot by getting inside your vehicle and getting near the parking spot. You will see a green marker. Drive into the parker and press E to park your vehicle. Parked vehicles are safe from robbery and will not be despawned from the server.