Marrying with an expired F-1 visa

I am an American citizen. My gf is an international student. She graduated and overstayed her visa. Her visa and I-20 are expired. We want to get married this December. Is it possible for us to marry with just her valid passport? What other documents do we need for this process? We really need help because I tried to research for this problem but they did not give me the accurate answer. Thank you so much!