Hopefully you kept a copy of your filing from last year.   If you used TurboTax last year, use the file created when starting you filing this year and the AGI is brought in.  If all you have is your paper copy (or the PDF) of last year's filing, the AGI is on line item 7 of your Form 1040.

You can get a copy of your prior-year tax return, from TurboTax it will have your AGI. 

This link  How do I access a prior year return? has directions on how to get a copy of a prior-year tax return from TurboTax.

Also, you can request a transcript of your 2018 tax returns from the IRS.  The transcript will have your AGI.

This link to the IRS web site  Welcome to Get Transcript  has directions on how to obtain a transcript of your tax return.  

2018 agi number

 Please see the below TurboTax FAQ for options on how you can find last year's AGI:

https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/00/25947