The IRS requires your 2018 AGI to verify your identity for e-filing. If you're paper-filing your return, you won't need your AGI because those are manually processed.
The best place to get last year's AGI is from the 1040 form you filed for 2018. Look on page 2, line 7. If you filed a 1040NR, it'll be on line 35. You won't find your AGI on your W-2 or 1099 form because those forms don't take into account over a dozen above-the-line deductions that go into calculating your AGI.
If you're having trouble locating your 2018 return, the second-best place to get your AGI is from the IRS. You can order a free transcript of your return at the IRS Get Transcript site or you can purchase a full copy of your 2018 return. Both versions will include your AGI.
If you didn't file a 2018 federal tax return (or you haven't filed one yet), enter 0 as your AGI.