Hi, this is Jeff's wife, Rita. To the best of my knowledge, the company or the broker through which you trade will send you a Form 1099, listing all your transactions throughout the year, capital gain and losses. If it's short-term trading, usually under 2 months, than the income will be taxed at your ordinary tax rate, depending on what tax bracket you are in. If it's long-term and eligible capital gain (the U.S. company), than it will be taxed at a lower rate - 0%, 10%, or 15%, depending on your tax bracket. However, next year there might be some changes because of the Trump's tax law.
You will know how much taxes you owe when you file the tax return.
I do some investing through Schwab; they send me Form 1099 every year. Then I do the taxes with TurboTax.