marriage to immigrant within country

Florida,my girl friend and i are thinking about getting married, what forms, costs, and difficulties will or can we expect to encounter? also, she is here on a student visa and has been living hear for one year, will marriage affect her ability to stay and study uninterrupted and will her status need to be changed and if so what status should she apply for and how hard is it to get it?

You can marry at the local courthouse. As long as the marriage is spontaneous and was not planned before she came to the USA, you are free to marry.