What sort of legal formalities to start a rent a car business in india?

First you should see you vehicle are registered for commercial uses, this issue will be solved with the help of district RTO office of your city. Further plying of Tata you need a trained driver with valid liecence. You also need to decide a firm name and a current bank account in the name of firm. With this preparation you can start your firm in proprietorship. If you have not PAN then also take a PAN. This will be needed in future at the time of filling your income tax return on annual basis.