If my fiancé came here legally, now we are getting married and want to get him a green card through me, how do we go about this once married?

Generally, if a foreign national has entered the U.S. lawfully and with inspection, and he later becomes married to a U.S. citizen, the couple may successfully pursue a marriage-based adjustment of status application process. This is true even if the foreign national may have long overstayed an initial visa. The application process includes multiple steps, including filing properly prepared and fully documented Petition for Relative Alien, Application for Adjustment of Status (with an application for an Employment Authorization Document), biographical information forms, an Affidavit of Support by the U.S. citizen spouse (and by a joint sponsor if the U.S. citizen spouse does not have sufficient documentable income, etc. It would be wise for you and your fiance to consult with an immigration lawyer who, after learning all of the relevant information about your fiance, about his immigration-related history, about you, etc., would be able to advise about immigration eligibilities, options and strategies, and could offer legal representation in the often quite complex application process.