Just getting started at 18yo, need help setting up

Hello, recently I've started finding interest in investing and setting myself up for the future, and even short term. As of now, I already have a Fidelity Roth IRA account with some money in index funds that I plan to contribute to each month, but I am looking to start investing for short-term gains. I'm not too sure what the best way of doing this is yet. So I'd like to know if someone could please explain? Do I make another account on Fidelity for short-term investments? Do I use a different c