1) There is no need to declare purchase of shares but if your taxable income exceeds Rs 25 Lakhs, you need to disclose your assets and liabilities.
2) The long term capital gains (LTCG) on equity shares purchased and sold through stock exchange are exempt from tax. If the shares are held for more than 1 year, the profit/loss on sale of such shares will be LTCG. The purchase of shares of another company or even same company has no impact.
3) If you are selling the shares within 1 year, the profit/loss will be short term capital gains/losses.
4) As you are selling the shares after 1 year it will be LTCG only and exempt from tax, if you have transacted through stock exchange.
You need to file IT Return if your gross total income is more than the basic exemption limit of Rs. 2.5 Lakhs. You need to show income from all the sources. The agricultural income will also have to be shown as exempt income.