Do not seek work authorization if you are a U.S. citizen or permanent resident (green card holder). You do not require it. However, you must still be able to prove your status in order to work.
Apply for employment authorization if you are in the U.S. on a student visa (F-1 or M-1). You are eligible to apply for temporary work authorization either during your time as a student, or as part of practical training after you graduate. In this case, you must still apply separately for employment authorization (I-765) and be approved before you can work. There are certain limitations to this employment that you should understand before beginning work. Consult the U.S. Citizenship and Immigration Services website, or an attorney, for more information.
Find a company willing to sponsor you for employment. In this case, you must have specialized knowledge or abilities that the company is unable to find among local candidates. In this situation, you must not only apply for work authorization, but you must also apply for a specific type of visa (H-1B) which is intended just for this purpose. This is intended as a temporary visa, which can be renewed only once. After that, if your employer wishes to continue employing you, it must sponsor an application for you to become a permanent resident.
Apply for employment authorization if you are the dependent or spouse of a U.S. citizen, and are in the process of applying for permanent resident status. In this situation, you may be eligible to apply for employment authorization to work during the interim.
Apply for employment authorization if you are in the U.S. as a refugee or are seeking asylum. Most asylum seekers are eligible to apply.
Consult the USCIS website or an immigration attorney for information on several other more narrow categories of eligibility. This includes citizens of certain countries, and beneficiaries of certain treaties that the US is a part of.
Fill out form I-765 (can be found on the USCIS website). Applicants of certain categories may also be eligible to file online.
Carefully assemble all the necessary documentation, as specified in the instructions, and send the application to the appropriate address. Generally speaking, the more thorough and organized you are in assembling the documentation, the more rapidly your application will be processed. Even if an attorney for your employer is doing all the paperwork, you should check the forms and information carefully to make sure there are no mistakes.
Wait. This is the hard part. The waiting period varies widely, depending on the current workload. On the USCIS website there is a way to register so that you can check the status of your application online. Internet discussion forums can also be a good source of information about the average waiting period for applicants in different areas/categories.
