If you have not filed a 2019 tax return and you are not required to file a tax return and you do not receive Social Security benefits, then use this TurboTax website to register for the stimulus - https://turbotax.intuit.com/stimulus-check/
If you have filed a 2019 tax return and need to change or add your direct deposit information, see this IRS website - https://www.irs.gov/newsroom/economic-impact-payments-what-you-need-to-know
The IRS does not have my direct deposit information. What can I do?
In the coming weeks, Treasury plans to develop a web-based portal for individuals to provide their banking information to the IRS online, so that individuals can receive payments immediately as opposed to checks in the mail.
If you receive Social Security benefits, see this US Treasury website - https://home.treasury.gov/news/press-releases/sm967