If my fiancé and I want to get married, she has two children, how much money she has to be making in other to sponsor me?

That will depend upon when you will file for an immigrant visa. The salary changes every spring for form I-864 purposes, but is likely over $25,000, perhaps more. There may be a possibility for a "joint sponsor" to come forward. The process of obtaining a marriage based immigration visa is complicated. For that reason, I strongly recommend an appointment with a competent and experienced immigration attorney before there are any complications. Good luck.