If you were were married last year you have two options.

Married Filling Joint or Married Filing Separate. 

In almost all situations Married Filing Joint is the preferred status and if you choose this option you include both incomes.

Going forward, if you are divorce and have a dependent, then you should review Head of Household status.  If you qualify it would be a better option the filing Single.

Here’s a list of the filing statuses:

1. Single. This status normally applies if you aren’t married. It applies if you are divorced or legally separated under state law.
    
2. Married Filing Jointly. If you’re married, you and your spouse can file a joint tax return. If your spouse died in 2015, you can often file a joint return for that year.
    
3. Married Filing Separately. A married couple can choose to file two separate tax returns. This may benefit you if it results in less tax owed than if you file a joint tax return. You may want to prepare your taxes both ways before you choose. You can also use it if you want to be responsible only for your own tax.
    
4. Head of Household. In most cases, this status applies if you are not married, but there are some special rules. For example, you must have paid more than half the cost of keeping up a home for yourself and a qualifying person. Don’t choose this status by mistake. Be sure to check all the rules.
    

Are you asking what AGI to use for your 2020 return so that you can e-file?    If you filed a joint return for 2019 the amount on line 8b is still your 2019 AGI even though you are not married now.   If you do not have a copy of that return you can try to use zero.  If zero will not let you e-file you will need to print sign and mail your 2020 return.

You do not put anything on your own return about your ex-spouse.