"Could you help me fix the car please?" I assume the context of this is asking a friend? If you mean you're taking the car to a Mechanic (i.e. someone who you're going to pay) you wouldn't ask them to "help" you fix the car. You would say "Can you fix the car for me?"

The second one is right because we say "(to) help someone DO something" therefore there is no gerund (-ing) or "(to) + infinitive" after "help", so it is "Could you help me fix the car please?" Good luck ^_^

Could you help me fixing my car?