If Turbo tax prefills in my 5 digit signature ID does that mean they retrieved it from last year and that is all I need? In that case do I not need my previous year AGI?

Yes, you still need your 2017 Adjusted Gross Income amount. TurboTax should automatically populate your 2017 AGI if it automatically populated you self-select PIN. Otherwise, see How do I find last year’s AGI?

The Self-Select PIN is a random 5-digit number that you enter as your electronic signature when e-filing your return. You could use a new number each year.

When I go through the steps for efiling - I don't see anywhere where it lists or asks for my previous year AGI - there is only a spot where it lists the 5 digit signature ID (prefilled) and birthday (prefilled).  Where would I find where it lists the AGI to make sure it is correct?  

use EITHER  the AGI or PIN. leave the other field empty.