Tax audit applicable on share trading?

I am an investor in stock market and also have an account on name of my wife where I do day trading in equity and derivatives.

I daily buy and sell shares and also play in future and options.

My yearly transactions goes in 200 crore. When you buy a lot normally we have to pay margin money and not entire amount of a lot.

For example if I am buying a future lot of Nifty, the trading value will be around Rs. 5,00,000 but I have to pay only margin of 15% around 75,000.

Now my question are:

1. How to calculate income tax on investment in shares?
2. How to calculate income tax on share trading (future and option)?
3. Whether tax audit will be applicable in case of trading in shares?
4. How to calculate turnover of share trading? Whether it is value of lot or my margin money?