You can look at your W2 copy. Or did you have self employment income?
Are you actually asking about your 2017 AGI because you need it to e-file your 2018 return?
Where do I find my AGI?
Your 2017 tax return shows your 2017 Adjusted Gross Income (AGI) If you filed a joint return then the AGI is the same for each of you.
Form 1040EZ line 4
Form 1040A line 21
Form 1040 line 37
If you are using the same TT account that you used last year, you should also be able to find this number in your Tax Timeline for 2017
If entering the amount from your 2017 tax return does not work, then try entering “0” instead. If that does not work and you still cannot e-file, then print, sign, and mail your tax return.
You can get a free transcript of your past return from the IRS:
https://www.irs.gov/individuals/get-transcript
Note: If you filed late or amended your return, your correct 2017 AGI may not be in the system.
https://ttlc.intuit.com/questions/1944348-how-do-i-print-and-mail-my-return-in-turbotax-online
Note: If you mail your federal return, you will also need to mail your state return. You will not be able to e-file your state return.