Is it ok if I marry a US citizen before 90 days and quit my University? I won't apply for green card yet. I will apply for green card after 90 days

The general rule is that if a foreign national enters the United States on a temporary visa and marries a US citizen within 90 days of arrival, there is a presumption that the immigrant concealed his intent upon entry. You could always get married before the 90 days, but your petition for a green card will receive stricter scrutiny. I suggest that you work with counsel to handle your case from start to finish so that the process goes smoothly. Some of us charge a reasonable flat fee to handle the case from start to finish with no hidden costs.