Suppose I invest Rs 10000 in a stocks of company A. After a bullish year my stocks are worth Rs 12,000.so I decide to sell it. Do I have to pay tax on that Rs 12,000 which I get after selling my stocks ?

In India you pay tax on the capital gain of the stock - that is, how much profit you made on the transaction. For stock trabnsactions that gain is equal to sales proceeds minus purchase cost, or 12000 - 10000 = 2000 Rs.