To drive a vehicle, first open your Inventory, and click on the vehicle you want to drive. When you click on it, there will be a 'Drive' button. Click on this, and the vehicle will spawn in the region you are in. 

Once the vehicle is in the region, click on it and you will be driving the vehicle. To turn and accelerate, use the W,A,S,D keys or arrows keys and use the spacebar to brake. To get out of the vehicle, click on it. The vehicle will automatically despawn once you have exited it.