SSI is Supplemental Security Income. SSI is not reported on a tax return.
If you are not filing 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/
Social Security Retirement/Disability/Survivors benefits are reported on a form SSA-1099
If you receive Social Security benefits, see this US Treasury website - https://home.treasury.gov/news/press-releases/sm967
If you received a SSA 1099 form in January or February all of the information is already in their system and you do not have to do anything. If you did not receive a SSA 1099 form you will need to fill out a tax return for 2019 and include your direct deposit information, routing and account numbers. If you do have to file you can use TurboTax which has a free simple return available online.
If you had SSI (Supplemental Security Income) you will not receive an SSA-1099 and you do not file a tax return. Go to the TurboTax web site in DoninGa's answer and fill out the TurboTax stimulus payment form. Here it is again: https://turbotax.intuit.com/stimulus-check/