How to save for retirement when I am not working?

If you're not working, and you're single, then you cannot contribution to a retirement account.

However, if you're married, as long as one spouse has earned (non-passive) compensation, then that working spouse can contribute up to the lessor of $6,500 ($7,500 if 50 or older) or the amount of their earned income into an IRA in the name of the non-working spouse. As of 2020, there is no longer a maximum age for IRA contributions, but you must file your tax return as married filing jointly.

If the working spouse has a retirement plan at work, including SEP IRAs, then the deduction for non-working spouses' IRA contributions is reduced if your AGI is more than $116,000 and the deduction is disallowed if your AGI is more than $136,000.

If the working spouse has no retirement plan at work, then both spouses can contribute in total $13,000 ($15,000 if 50 or older) and the deduction for both IRA contributions are reduced if your AGI is more than $218,000 and the deductions are disallowed if your AGI is more than $228,000.