Once you decide which broker is best for you, all you need to do is fill out its online application and deposit money, either through a transfer from your bank (usually the easiest method), mailing a check, or transferring assets from another brokerage account. You can open an account in as little as 5 or 10 minutes on your phone or laptop. Some brokers won’t require you to wait until your ACH-transferred money clears before you begin using the account.