You are a permanent resident from the moment you enter
When CBP admits you with an immigrant visa, you become a lawful permanent resident that day. The physical green card comes later by mail.
CBP stamps your passport, on the immigrant visa page. This visa plus the stamp is a temporary I-551, valid as proof of permanent residence for 1 year from the date you entered. You can use it to:
Start a job (show the passport with the visa and stamp).
Travel abroad and come back during that year.
Apply for a Social Security number.
Pay the USCIS Immigrant Fee
USCIS will not make your green card until the fee is paid.
The fee is $235 per person. Some people pay $0: children adopted through the orphan or Hague programs, Iraqi and Afghan special immigrants, returning residents (SB-1) and K visa holders.
Open uscis.gov/uscis-immigrant-fee and choose "Pay my USCIS Immigrant Fee".
You need your A-Number and DOS Case ID. Both are on the immigrant visa data summary sheet or the visa itself.
Pay online by card or US bank account. Anyone may pay for you.
USCIS recommends paying after you get the visa and before you travel. You can still pay after arrival.
Track it in your USCIS online account or on the case status page. If the card has not arrived 90 days after you paid the fee, contact USCIS through its online help form or e-Request tool.
Not paying does not cancel your status. But without the card, your temporary proof ends after one year, and life gets hard: travel, jobs and IDs all ask for it.
Give a correct mailing address with an apartment number. Update USCIS if you move.
Many green card holders get a Social Security card automatically because they asked for it in the visa application. If you did not, apply. See How to get an SSN: students, workers and spouses.
Men 18 to 25 must register with Selective Service. See that guide.
Forgetting to pay. It is the main reason cards do not come.
Letting the 1-year stamp expire without a card. Contact USCIS well before.