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Investing in stocks with gross income (not yet taxed) cash from contract work?
You need to report the income from any work as income, regardless of if you invest it, spend it, or put it in your mattress (ignoring tax advantaged accounts like 401ks).  You then also need to report any realized gains or losses from non-tax advantaged accounts, as well as any dividends received.  Gains and losses are realized when you actually sell, and is the difference between the price you bought for, and the price you sold for.  Gains are taxed at the capital gains rate, either short-term or long-term depending on how long you owned the stock.   The tax system is complex, and these are just the general rules.  There are lots of complications and special situations, some things are different depending on how much you make, etc.  The IRS has all of the forms and rules online.  You might also consider having a professional do you taxes the first time, just to ensure that they are done correctly.  You can then use that as an example in future years.