Here is how:
There are three ways.
If you have filed your last year 2018 tax return and have a copy of the tax forms, you may locate the amounts on line 7 on your 2018 Form 1040 or line 35 on 2018 Form 1040-NR if applies to you.
If you used TurboTax online to file, you can access your account here Login and download a copy of your prior year’s 1040 tax return to find your AGI. Click here for instructions: Prior year's return
If you do not have a copy of the tax forms, you may order a transcript from the IRS. Click here Get Transcript and select "Get Transcript Online " On the transcript, look under section "Adjustment to Income" and find the amount for Adjusted Gross Income.
You can find it on the printed copy of your 2018 return.
If your filing status changed, then it is more complicated.
Here is an article about what to report for Adjusted Gross Income if the filing status changed between Married Filing Joint and Married Filing Separate.
Check this resource which explains what to do if you did not file last year (use -0-) or if you amended last year (use the originally filed AGI): Where do I find last year's AGI?