A green card is basically a permanent citizenship card issued by the US Citizenship and Immigration Services (USCIS). Here are the things involved.
1: Well, first things first, get married! Yes, get married to an American citizen! Okay, but it is not a cakewalk after that. The wedding should be legally perfect and viable. That is you should be able to produce a proof that it is a bona fide marriage. The immigration department will not ignore sham marriages. Ultimately, your marriage registration and related documents should prove that you did not get married just to receive an immigration benefit.
2: Taking the earlier point ahead, mentioning of fraud while getting married, if anyone goes to the USA on a visitor visa thinking that he or she can get a green card right away after entering the country, this is false conception. According to the US law regarding this, if anyone gets married before 60 days post entering the country and after 30 days of entering the country, it is automatically assumed as a fraud. As then, it seems evident that the person entered the country with an intention of getting immigration benefits and not to visit. Please keep this is mind.
3: Now, get the documents in order. The immigrant spouse has to submit two passport style photographs, a copy of his or her birth certificate and a copy of his or her non-immigrant visa (or other proof of legal US status). The application should also include an Affidavit of Support accompanied by recent tax returns and an employment verification letter and proof of AOS eligibility. That will be the marriage certificate and divorce decrees (if at all) from previous marriages for both parties.
4: The next thing to do is to complete a medical exam done conducted and mandatory under immigration laws. The doctor has to be one approved by the USCIS. A sealed document containing the results of this exam given to you by the doctor has to be attached to the I-485 application.
5: While filing the application do not forget to fill the following forms: I-130, I-485, and G-325A applying for both the US citizen and the immigrant spouse respectively. Include the appropriate fees for the forms. These forms are for petition for alien relative, application for adjustment of status and also biographical information form. It safe to fill these forms in case you are already in the USA and have another proper resident immigration status.
6: If you are planning to work in the US, file form I-765 to get a work permit while you are carrying out the application. This application will be processed in around 90 days.
7: When you are given a fingerprint appointment, never ever miss it. Once you give the application for a green card, an appointment intimation would be received by you. The appointment would be scheduled between 1 to 3 months of your filing the application. Do not for any reason miss this, as your application would be rejected in that case.
8: The interview includes the interview for Adjustment of Status. Don't think of missing this even! An appointment would be received you which would be scheduled anytime between 6 months to a year. (It is certainly going to help you develop a great virtue, patience!). There you need every possible proof you might have of your bona fide marriage.
Remember that if your visitor visa has expired and you file the forms, you are in real trouble. So, never ever forget this. Ultimately, no one can say how much time the entire process will take. Just wait and watch. All the best!