I want to try swing trading with my long-term stocks. How can I ensure that I only sell the new, short-term shares and don't accidentally sell my older, lower-tax long-term units? What is the default rule that my broker uses?

I want to invest in the US stock market or in S&P 500 ETFs for approx. 15 years. Could you please suggest some platforms that offer long-term security for my funds, along with their charging details?