18岁刚起步，求教如何设置账户

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 company (I heard Webull and Robinhood are good) for these investments? Thank you