Can I file 2018 taxes first I don't remember who I filed with previously

Yes, you can.  In order to obtain the 2017 AGI:

If you have an original copy of the 2017 tax return, check here  2017 AGI.

Or 

The best and fastest way to access the 2017 AGI is to order a transcript from the IRS.  Click here  Get Transcript  and select "Get Transcript Online " On the transcript, look under section "Adjustment to Income" and find the amount for Adjusted Gross Income.