If I'm retired what form do I use no job just a pension from previous employer and social security? Do I have to file im low income?

You may have to file if your taxable income from the pension and your taxable Social Security benefits is greater than the Standard Deduction for your filing status -

Standard deductions for 2021

Single - $12,550 add $1,700 if age 65 or older
Married Filing Separately - $12,550 add $1,350 if age 65 or older
Married Filing Jointly - $25,100 add $1,350 for each spouse age 65 or older
Head of Household - $18,800 add $1,700 if age 65 or older

Up to 85% of Social Security Retirement/Disability/Survivors benefits becomes taxable when all your other income plus 1/2 your social security reaches:

Married Filing Jointly - $32,000
Single or Head of Household - $25,000
Married Filing Separately - 0

You can use the calculator at the following link on the IRS web site to see if you have to file a tax return.

Do I Need to File a Tax Return?