The US has several layers of government. Knowing which one handles a task saves you from waiting in the wrong line.
Immigration: USCIS (applications), CBP (borders, I-94), ICE (enforcement), the State Department (visas, through US embassies abroad).
Social Security numbers: the Social Security Administration (SSA).
Mail: the US Postal Service.
Passports for US citizens.
Driver's licenses and state ID: the DMV (the name differs by state).
State income tax (most states, not all).
Professional licenses (nurses, teachers, engineers, electricians).
Unemployment benefits, Medicaid and SNAP (run by the state with federal money).
Most criminal and traffic laws. This differs by state.
Courts, jury duty and the county clerk (marriage licenses, property records).
Public health department: vaccines, WIC, clinics.
Property tax and records. Sometimes the sheriff and jail.
Trash and recycling, water, streets, parks, libraries.
Police and fire departments.
Building permits, noise rules, parking tickets.
Public schools are run by a local school district, which can cross city lines.
Many cities have 311: a number (and often an app or website) for non-emergency city services. Use it for:
A missed trash pickup, a broken streetlight, a pothole.
Noise complaints, illegal dumping, a dead animal on the road.
Questions like "When is bulk trash pickup?"
If your city has no 311, call the city hall's main number or use its website. For emergencies, always call 911.
Go to usa.gov and search your task. It links to the right federal, state or local office.
Search your state name plus the task, for example "Ohio driver's license". Official sites end in .gov (some states use .us).
Avoid paid "help" websites that look official. They charge for free forms (see the guide on fake government websites).
Asking the DMV about your visa, or USCIS about your driver's license.
Paying a website to "process" a free government form.
Calling 911 for a pothole or noise. Use 311 or the non-emergency police number.