If you filed your own return last year, but someone also claimed you as a dependent, use the AGI (Adjusted Gross Income) from your return.

If you can not access last years return then you can:

1. View or download a transcript of your return online at www.irs.gov.
2. Go to www.irs.gov and request a hard copy transcript of your return be mailed to you.
3. Call the IRS at 800-908-9946 and request a hard copy transcript be mailed to you.

If you didn't file a return last year just indicate that you did not file and don't have AGI.

does that mean enter $0? I didn't see a choice besides entering a number. 

If you didn't file a return last year (includes filing for the first time), enter "0" as your AGI.

If I didn't file a 2019 tax return and was claimed as a dependent on a parent's income but received income from a job is my AGI 0?

That's right -- if you didn't file last year, your AGI is zero.  

If you had filed last year, the IRS would require your AGI from last year to verify your identity in the electronic filing process.