Building vehicles is very simple in ROBLOX. A basic car needs just a seat, body, and wheels.
Insert a new Part for the main body of the car. Use the Scale tool to make it longer and taller.
Insert a Cylinder for the first wheel. Use the Surface tool to make one of the sides a hinge.
Make sure the cylinder is right next to the car body! Otherwise the wheel will fall off.
Duplicate the wheel so you have 4 wheels in total. Use the Rotate tool to make sure the hinge of the wheel faces the body of the car.
Raise the car body with the Move tool so it is off of the ground.
Last, we need a VehicleSeat. This is where the player will sit to drive the car. Right click in the 3D view, hover over Insert Object, then select VehicleSeat.
Move the VehicleSeat so it is on top of the car body.
We now have a working car! You can now press Play, jump on the VehicleSeat, and drive your car around!