Car Garage Services Under Gst

Dear Sir, my car machanic (Car Garage) is, I want to take GST, what will be the highest GST scheme for me?

As I am a car owner, I come in the service provider's service, I make the same purchase in the customer's car and I also bill your service with the purchase of that item, then I only deposit the tax on the service.
I just tax the service bill. Make a tax by making a bill with 6% composition scheme or the same which has been imposed.