A green card through marriage to a US citizen is the most common way to become a permanent resident. The spouse of a US citizen is an “immediate relative”. This means that there is no limit on the number of people who can obtain green cards through marriage to US citizens.

The US citizen starts the process by submitting a form I-130 visa petition on behalf of his/her spouse. If the spouse entered the US lawfully, he/she can file for Adjustment of Status (Form I-485) at the same time and get a green card without having to leave the US.

Helpful information for those seeking a green card through marriage is available at:
Tips on How to Prepare for Your Green Card Marriage Interview
Green Card Marriage Questions
If the marriage is less than two years old when the green card is granted, it will expire in two years. The couple must submit a form I-751 joint petition during the 90-day period prior before the expiration of the green card. This way, the foreign-born spouse can obtain a ten-year green card.