I moved to US in the year 2018 and I am filing my tax return for the year 2018 as a resident alien. While filing for an extension, what value should I fill for AGI?

When filing an extension, you will estimate the amount of your income by referring to any income statements that you may have received in 2018 (e.g. Form W-2, Form 1099-MISC, payroll statements, checks, etc.). Any adjustments to income such as IRA deductions, alimony, educators expenses, are then subtracted from your income.  

I have attached the Form 1040 Schedule 1, Additional Income and Adjustments to Income. The adjustments to income begin on line 23 of the form.

What if I had absolutely no US income in the year 2017 and only had income in a foreign country where I was residing/working?

If you were not considered a resident alien of the United States in 2017, then you were not required to report your worldwide income.  You will enter -0- as the AGI amount for tax year 2017.