I am trying to sell stocks in order to put a down payment on a house and I want to sell enough to cover it but I am unsure of how it should be taxed so that I'm not left with a massive payment when I file my taxes.
It depends upon your filing status as well as any other sources of income you may have. Pennsylvania does not have any separate capital gains rate from short-term or long-term capital gains. Therefore, any sales of stocks will be taxed at the ordinary income tax rate within Pennsylvania. Please see this link for guidance on calculating the amount of tax that would be due on your stock sales. Remember, you would only pay tax on the actual gain amount (sales proceeds amount that exceeds your cost basis in the stock).
Also see Form Rev-414(I) for the 2025 estimated tax worksheet to assist as well as this link which provides instructions on how to pay your estimated taxes on any additional income to Pennsylvania.