Will I owe taxes when I sell a stock or mutual fund?

From FinancialPlanning

If the stock or mutual fund is held within an IRA or similar tax-deferred account, no tax is owed at the time of sale. Otherwise, you'll need to compute the gain or loss on the transaction, account for it on your income tax return, and pay the taxes owed.

Whether you need to pay any taxes immediately (or rather, at the time of the next quarterly estimated tax payment due date) depends on the amount of the gain. It may be acceptable to pay the tax at the time you file your income tax return for the year of the sale.

See the FAQ page on capital gains tax rates for further details on the tax owed. See IRS Publication 505 for information on figuring out whether you need to make estimated tax payments because of a large gain from the sale of stock or mutual funds.