Internal Revenue Service (the IRS)
www.irs.gov
Here you will find information related to the federal income tax. You may find "Publication 17 - Your Federal Income Tax" and "Publication 919 - How Do I Adjust My Tax Withholding" of some interest.
Social Security Administration
www.ssa.gov
Here you will find information related to the social security tax, as well as information about your social security card and number. Social Security is based on a simple concept: While you work, you pay taxes into the Social Security system, and when you retire or become disabled, you, your spouse and your dependent children receive monthly benefits that are based on your reported earnings.
U.S. Citizenship and Immigration Services
www.uscis.gov
U.S. Citizenship and Immigration Services (USCIS) is the government
agency that oversees lawful immigration to the United States. Here you can find information about working in the USA if you are from another country.