Investment accounts can be opened at various financial institutions. Below are links for 4 of the largest institutions. Click on the link then follow the steps to open whatever account you want.
(See also: Q&A: What type of investment account should I open?)
Charles Schwab
https://www.schwab.com/public/schwab/investing/accounts_products/accounts/open_an_account.html
Fidelity
https://www.fidelity.com/open-account/overview
TD Ameritrade
https://start.tdameritrade.com/select?entity=103
