If your total income from all sources for the 2019 tax year is less than $12,300 you do not need to bother filing a tax return.
However, if you have self-employment income of more than $400, then you are required to file a tax return. With only alimony income, you would not be required to file a return at all. Though I do question how you survived the entire tax year on only $5,250.
To register for the stimulus without filing at tax return, go to https://www.irs.gov/coronavirus/non-filers-enter-payment-info-here
