If you did not file a tax return for 2017 at all, then use zero for the AGI in order to e-file your 2018 return. If zero does not work then you have to print, sign and mail your 2018 return.
https://ttlc.intuit.com/questions/4501187-where-can-i-find-my-prior-year-2017-return
You can get a free transcript from the IRS:
https://www.irs.gov/individuals/get-transcript
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