I can not find my return from last year and do not know my AGI for 2017, How can I sign in to last yyrs taxes or can you tell me how to figure it out from w2 and medicare

I am filing for 2018, I have 2017 w-2s and medicare paperwork. Why do I have to enter last years to get this years? If you tell me what you want added together to get AGI for 2017 I can do it.