The I-94 is an arrival and departure record issued by the Department of Homeland Security to aliens admitted to the US.
You will not need your I-94 to start work, but you might need it for official business such as going to the DMV or the Social Security office.
Here is how you can find and download your I-94:
1) Go to the US Customs and Border Protection website.
2) Click "Get Most Recent I-94"
3) Acknowledge the Terms of Service
4) Enter your Traveler Info (Name, Date of Birth, Country of Citizenship, and passport number)
5) Download and print/save your I-94 for your records.