Immigrant visas are valid for 6 months after the interview date. You must enter the US within that duration. You become a permanent resident only when you enter the US. Until then, you just have immigrant visa and you don't have any particular status in US just because you have an immigrant visa.

You don't need to fill I-94 form in the flight. Just the customs form. You enter US with your immigrant visa, passport, the sealed blue envelope given by the hospital at the time of the medical examination and the sealed envelope given by the consulate. At the port of entry, you will be taken to a special room (or there may be a separate section for new immigrants, depending upon the airport) and your passport will be stamped with I-551 stamp which is a temporary green card, valid for one year. It indicates the date and port of entry for admission as a lawful permanent resident of the United States. You will get the actual plastic green card (which looks just like driver's license in US) in the mail within few months. At port-of-entry, you will also be fingerprinted (only right hand index finger) which just takes couple of minutes. When asked to sign, remember to sign in black pen. If you are in EB3, for example, your passport will be stamped with E31, your spouse E34 and your children E35. Similar numbers for other categories. Temporary green card is as good as permanent green card except that it expires in one year.

If you were in US previously and had had already applied for I-485, the A# number in your I551 stamp should be from immigrant visa, and not from I-485/EAD. In actual plastic green card, the A# number will be from immigrant visa.

If the immigration officer detects anything wrong in your documents or with your answers to his/her questions, he/she can use expedited removal powers to send you back home and bar you from entering the US for 5 years.

Marriage based immigration:
If your marriage will be less than 2 years old when you enter the US as new immigrant, you will be admitted into the US as conditional resident and not permanent residence. 90 days before 2-year anniversary, you will have to apply to remove conditions to get permanent residence. Therefore, if your 2-year anniversary is close (before 6-months immigrant visa expires), wait until after your 2-year anniversary to enter the US to receive permanent residency directly. Make sure to point to the immigration officer at the port of entry so that he/she stamps your passport for permanent, not conditional, residence.

Welcome to United States as a permanent resident. Your plastic green card would arrive in your mail by normal US mail within 2-3 months. It would come from the USCIS service center. On the envelope, some USCIS service centers write that do not forward. So if you change your address between the time you enter as immigrant and you would hopefully get the plastic card, you should promptly file Change of Address Form (Form AR-11) to USCIS and follow up with them to make sure that they have your current mailing address.

Sample USCIS envelope containing permanent resident card

Before getting immigrant visa, if you had been working in US, you would normally have non-immigrant visa such as H1B, L1 etc (some of you might even be working on EAD if you are on H4, L2 etc.), those non-immigrant visas would have got cancelled at the time of consular interview. So they are no longer valid to work. So you should ask your employer to give you I-9 form (Employment Eligibility Verification Form) and you should fill it. Give them the copy of your I-551 stamp in your passport. Now you can legally work with your employer based on your green card.

When the person is on non-immigrant visa, he/she would have either social security card with "Valid to Work with USCIS Authorization only", or "Not Valid to Work" or no social security card at all. You should go to nearest Social Security Administration office and get a new Social Security Card. You can fill out the fillable form SS-5 and print before you go there. You will get new Social Security Card with those restrictions removed. You will retain the same old Social Security Number, though.

Newcomer's guide to USA