How do I get a green card, if I’m from Serbia? Serbia has changed its passports and I do not have a passport to even get back into my country.

The question

I came to America in 1996. I got married, but my husband passed away in 2018. We had a son together and I’m still carrying his last name. Now, I have met someone and we want to get married. I need a green card, so I can renew my driver’s license and to be able to work. Right now I’m a caregiver to a lady who suffers with 24/7 Neuropathic pain syndrome. What do I need to do to become legal in the United States of America?