If you have filed a tax return for 2018 or 2019, you will automatically receive a stimulus check (or direct deposit if you have provided your bank information to the IRS on a tax return) if your AGI is $75,000 or less ($150,000 if married filing jointly). The amount of the stimulus will decrease until your AGI reaches $99,000 ($198,000 if married filing jointly)
Please read this IRS Q&A page for more information.
If you receive Social security benefits and are not required to file a tax return, you do not need to do anything. The IRS will direct deposit your stimulus payment into the account receiving your benefits.
If you are not required to file a tax return for 2019 because you have no income or your income is below the filing thresholds, you can register for the stimulus payment on this IRS web portal.