why do my car shake?

Do you mean the vehicles shakes like brake pulsation you feel in the brake pedal and steering wheel? That would indicate the front brake rotors may be out of true and may need to be machined or replaced.If the engine shakes and vibrates and or stalls, it may indicate a misfire due to loss of ignition or fuel which should bring on a check engine light. If the yellow check engine light is on fault codes will be stored in the computer of your car that would indicate where the problem is but you would need a scan tool to retrieve codes stored.