The US citizen can petition you for an immigrant visa, if that’s what you mean. I suggest she starts by joining Visa Journey, so that you both can start learning about the process.
It is not fast, or cheap. It will take several years, and you will have to provide a lot of evidence in terms of ‘in-person’ meetings, because Tunisia is a high fraud/high risk country. Good luck.
There are guides at the top of the page to help, as well as a MENA page/Tunisia portal for more specific advice.