Step 1.
Make a car, and some ground to drive on, and add a brush with the texture "Origin" where I have put it:



Step 2.
Now you make a path_track entity in front of the car (this one shows where the car will start), and another one a bit further away (this one shows in what direction the car will start), like I have done:



Set these properties for the closest path_track to the car:

Name Track1
Next Stop Target Track2

Set these properties for the second path_track:

Name Track2
Next Stop Target Track1

You do not have to bother with the other properties.

Step 3.
Now you mark the WHOLE car, every tiny brush of the car, plus the Origin brush and tie them to the entity func_vehicle.

I recommend these properties:

Name Car1
First Stop Target (The name of the first path_track)
Length of vehicle (The length of the vehicle in units)
Width of vehicle (The width of the vehicle in units)
Height above track (Try different values, it depends on how tall you made the Origin brush)
Speed 500
Damage on crush (How hurt you will be after being hit by a moving car)

Step 4.
Now, you've made the car, but no any entity to start it! Make a brush with the texture AAATrigger (it doesn't matter what texture you use, but it's quite a distinctive looking texture in the editor) exactly where you want to steer the car. For example, in real life, it would be best to put the brush behind the steering wheel. When you have added it, tie it to the entity func_vehiclecontrols.
Use these properties:

Name (The name of the car)

Now jump into your new Hummer, and cruise around in the terrorist environment.