As a routine matter, U.S. Customs and Border Protection may search your belongings even if you don’t declare anything. The full list of customs rules is at cbp.gov, but some frequently asked questions include:

What may I bring in duty-free?

What are the rules regarding large amounts of money?
Anyone traveling with $10,000 or more in cash—whether it’s in U.S. dollars or any other currency—must declare it.

What are common mistakes travelers make?
Some items will be confiscated if you try to import them, such as counterfeit goods; Cuban cigars; most agricultural (plant and meat) products; and drug-related equipment. The presence of any illegal drugs will result in prosecution.

Your airline will give you documents to complete while en route to the USA. All travelers are required to complete Customs Declaration Form 6059B. Those travelers who are non–U.S. citizens and are requesting admission to the USA with a visa will also be handed Form I-94 (white), Arrival/Departure Record.

You must go through U.S. Customs and Border Protection (CBP), for immigration and customs processing. CBP must approve your entry upon your first airport landing in the USA.

At Passport Control, most foreign travelers need go through the US-VISIT process. A CBP officer takes your electronic fingerprints and a photograph as part of your travel record. US-VISIT uses biometrics—such as fingerprints—to establish and verify your identity and identification materials and to check you against a watch list of known or suspected terrorists, criminals and immigration violators. You may be asked questions such as what you do for a living, whether you have enough money for your visit (a credit card is a smart precaution), and when and how you will be leaving the country (bring a copy of your itinerary). It is advisable that single parents and guardians traveling with a minor should have proof of legal custody.

After Passport Control, gather any baggage and enter Customs.

Every visitor entering the USA from abroad needs a passport. Foreign passports must be valid for at least six months longer than your intended stay in the USA unless your country of origin is exempt from that requirement – see the exempt countries at this CBP website.

Foreign nationals entering the U.S. are also required to present a valid visa issued by a U.S. consular official unless they are:

A foreign national traveling by air who is a citizen of a country eligible for the Visa Waiver Program must have an approved ESTA (Electronic System for Travel Authorization) before traveling to the USA. You may apply for ESTA at esta.cbp.dhs.gov.

If you are a citizen of a Visa Waiver Program country (see section below) and your passport was issued before October 26, 2005, it must be “machine readable” (with two lines of letters, numbers and <<< at the bottom); if it was issued between October 26, 2005, and October 25, 2006, it must be machine readable and include a digital photo; and if it was issued on or after October 26, 2006, it must be an e-passport with a digital photo and an integrated radio frequency identification, or RFID, chip containing biometric data.

Canadian citizens traveling from Canada, Mexico, the Caribbean or Bermuda, by land or sea, will be required to present one of the following travel documents: Passport, Enhanced Driver's licence, Trusted Traveler Card (NEXUS, SENTRI or FAST), or Secure Certificate of Indian Status.

Exceptions to the requirements include:
Canadian citizens at pre-clearance Canadian airports may present a NEXUS card at a NEXUS kiosk.
USA Lawful Permanent Residents are required to present a Permanent Resident Card or other evidence of permanent resident status.
Canadian citizen children under age 16 arriving by land or sea from Canada or Mexico may also present an original or copy of his or her birth certificate, a Consular Report of Birth Abroad or a Canadian Citizenship Card.
Canadian citizen children under age 19 arriving by land or sea from Canada or Mexico and traveling with a school group, religious group, social or cultural organization, or sports team, may also present an original or copy of his or her birth certificate, a Consular Report of Birth Abroad, or a Canadian Citizenship Card.

For more CBP travel information, go to cbp.gov.

Generally, a citizen of a foreign country who wishes to enter the USA must first obtain a nonimmigrant visa for a temporary stay. The visa allows you to travel to the USA port of entry and request permission of the USA immigration inspector to enter the country. For general information about applying for a visitor visa, including documentation and fee requirements, please visit travel.state.gov. For information on arrival procedures in the USA, including customs requirements, please visit cbp.gov.

What is a visa?
A visa is permission to apply to enter the USA. A U.S. consular officer will issue a visa after determining that the applicant is eligible to travel to the USA under a particular visa classification.

Who needs a visa?
A visitor (temporary) visa is required of all visitors seeking to enter the USA with the exception of nationals of Canada and countries that are part of the Visa Waiver Program. Currently, 36 countries participate in the Visa Waiver Program. For a list of participating countries and additional information on the Visa Waiver Program, visit travel.state.gov.

Applying for a visitor visa
Applicants for visitor visas generally should apply at the nearest U.S. embassy or consulate in the country where they live. As part of the visa application process, an interview at the embassy or consulate consular section is required for visa applicants from ages 14-79, with few exceptions. Persons age 13 and younger and age 80 and older do not require an interview, unless the embassy or consulate asks them to be interviewed.

You can complete a U.S. visa application online at travel.state.gov. Once you have completed the application, the next step is to make an appointment for a visa application interview. The wait time for an interview appointment for applicants can vary, so we strongly encourage you to apply as early as possible. Visa wait times for interview appointments and visa processing times for each U.S. embassy or consulate are available at travel.state.gov. To learn how to schedule an appointment for an interview, pay the application processing fee, review embassy specific instructions, and more, please visit the website of the U.S. embassy or consulate where you plan to apply. You can find a list of all U.S. embassy and consulate websites at usembassy.gov.

There are different categories of visas that correspond to your purpose of travel, and each one has its own application procedures. If you are traveling to the USA for a reason other than tourism or temporary business—for example, to study or for temporary employment—you will need the correct visa. You can find more information at travel.state.gov.

Further visa inquiries
Questions on embassy-specific visa application procedures and questions on visa ineligibilities should be addressed to the U.S. embassy or consulate where you will be applying. Please be sure to check travel.state.gov, which has complete information about the U.S. visa process. Very often you will find the information you need.

In order to enter by land from Canada or Mexico, certain documents are required beyond your passport and visa (if necessary):

Driver’s licence. (Canadian and Mexican licenses are enough, although an International Driving Permit is not a bad idea.)
Registration paperwork for the vehicle.
Proof of liability insurance.
The entry time varies depending on the border crossing. The current wait times are updated hourly on the U.S. Customs and Border Protection’s website at apps.cbp.gov/bwt.