I have not yet filed for 2019. In 2018, I had a negative AGI. Will I get a stimulus check from the CARES Act?

No, you must file something.  These links may help you.

https://turbotax.intuit.com/stimulus-check/ link to stimulus registration (SR).  It is very important that you uses this link that says register now if you don't need to file taxes this year.  Otherwise, you will not be able to E-file a zero return. It also has the calculator.

The IRS says it is working on developing a web-based system for people to give the government their direct-deposit information. Keep checking www.irs.gov/coronavirus for more details on this.

If you only want to change your Direct Deposit Information

The Treasury is expected to shortly announce an online portal where direct deposit information can be supplied to the IRS. Here is what you should do: 

Go to  http://www.irs.gov/newsroom/economic-impact-payments-what-you-need-to-know to see if they have set up the online portal. (This article will be updated with the web address once it’s available.)