Your AGI is your adjusted gross income.  If you have a copy of your prior year return (2017), you will find it on  form 1040 (Line 37), 1040A (Line 21), 1040EZ (Line 4), 1040NR (Line 36), Form 1040X (Line 1, column A).

If you did not file your 2017 returns with TurboTax, (and you do not have a copy of your prior year return) you can quickly download a free IRS transcript of your 2017 return from the IRS website which will include your accurate AGI. Select the blue link above then 'Get transcript online'. 

On your transcript, look for ADJUSTED GROSS INCOME (you may need to look on page 2).