If already married, you can apply for a marriage visa. This typically is a more complex process, takes longer, and costs more money.
However, with either a marriage or a K-1 visa, you can apply for a green card by filing Form I-485, Application to Register Permanent Residence, or Adjust Status. If you have been married for less than two years, you will receive a conditional green card valid for 24 months. You must then file Form I-751, Petition to Remove Conditions on Residence.