Hi Letoya,
You can find this figure on line 18b, on the first page of your 2018 Form 1040. If you used TurboTax last year, and the 2018 TurboTax data file is still on your hard drive, the 2019 TurboTax software SHOULD automatically import last year's tax data (including your 2018 AGI) into this year's version of the software as well -- so you can compare figures between this year and last. But your 2018 Form 1040 is the "official" answer. Hope this helps.