first of all make your vehicle, make it so it faces west, also you will need to put a brush in the middle of your vehicle with the texture "origin" which can be found in half-life.wad, tie all of the brushes that make the vehicle. Now you want to select the vehicle and hiy ctrl+T to tie it to an entity, select func_vehicle from the drop down list, name your vehicle "vehicle1", in "first stop target put "stop1". Now create the entity "path_track" this is were you want your vehicle to start, name it "stop1", then in the field "next stop target" put "stop2", create another path_track and name it "stop2" this is the direction you want your vehicle to face, for stop2 in the field "next stop target put "stop1". Now for the controls, put a brush were you want to control the vehicle from, remember that the player will have to stand into the brush to control the vehicle not next to it. Use the aaatrigger texture and tie it to entity, select func_vehiclecontrols from the list, in the only field put "vehicle1".

i hope i didnt forget anything but thats it... i think