There are several platforms you can utilize to trade on, but I am going to focus on one. I use eTrade for all transactions and use the free platform to do so.
You will need to open an account, connect it to your bank account, and when ready move money into your eTrade account. After you make money on eTrade you will then be able to transfer money back.
Set up an eTrade account
Go To eTrade and select "Open an account" in the top right corner of the screen
After selecting "Open account" follow the steps to create a login and password