Visit www.irs.gov and create an account to see your previous years transcripts online OR download the PDF version of 4506-T and mail it to the address provided OR call the 800 number and follow the instructions. Please note you will need your filing status and street address number or your PO Box number to request upto 10 years of transcripts via telephone.
You asked: "What can i do if i don't have 2018 W-2 to get AGI ?"
Your AGI is not on a W-2. Did you file a 2018 return, and if so, do you not have a copy of your 2018 return? If it was prepared in Online TurboTax, and if you are in the same account, there should be a direct link to the 2018 AGI in your online account. Or you can download the 2018 return PDF.
Or did you use Online TurboTax for 2018, but can't locate the account with your 2018 return? If you have multiple accounts, we can help you find the right one.
See this FAQ: How do I find last year's AGI?