As an immigrant, you do not have the immediate right to work in the United States. You will need to go through a process to secure the ability to seek employment.
U.S. Citizenship and Immigration Services explains you have three options for securing the right to work as an immigrant.
You can seek temporary worker status. This option requires you to have the employer for which you want to work file a petition for you. The employer will let the USCIS know about the employment it is offering you.
The permanent worker option is if you have special skills, experience, education or background that allows you to qualify for a job in the U.S. If you can offer something unique to the job field, you may be able to apply for an employment-based visa.
If you only want to work in the United States, you can get a work visa. You will not go through the USCIS for this. Instead, you must apply with the U.S. Department of State. However, you may need pre-approval from the USCIS prior to applying with the State Department. Even with the visa, you still must pass through U.S. Customs and Border Protection prior to entering the country.
If your status as an immigrant does not qualify you for any of these options, you will need to seek a change of status. You can either seek employment authorization or become a lawful permanent resident.
Employers must obtain documentation from you that shows you are legally able to work in the country prior to hiring you. If you cannot provide these items, then you likely will not qualify for the job.