Since you didn't file 2017 then put down 0 for the AGI to efile 2018. File each year separately. You can still efile 2018 until Oct 15. If you are getting a refund it's ok to file late. If you owe any penalty or interest the IRS will send you a bill.
The instruction sheet that prints with your return should say where to mail it. Where to mail federal paper returns
https://www.irs.gov/uac/where-to-file-paper-tax-returns-with-or-without-a-payment Don't forget your state. State goes to a different place.
And how did you prepare 2017? Online is only good for the current year 2018 so you would have to use the Desktop CD/Download program installed on a computer.
