I'm a US citizen and want to sponsor my spouse for a Green Card, but we're separated.

Consult with an immigration attorney to determine the impact of separation.

Gather documentation of your marriage and separation.

You can still sponsor your spouse for a Green Card, but you'll need to demonstrate that your marriage is bona fide and not a sham, and the separation may raise questions during the application process that you'll need to address

Be prepared to provide evidence of your relationship.