A notary public is a person licensed by the state to check your identity and watch you sign a document. They then add a stamp and signature. This is called notarizing. It proves that you signed, not that the document is true or legal.
You may need it for: a lease guarantee, a letter of consent for a child's travel, a car title, a power of attorney, some bank and school forms, and some documents for your home country.
A notary in the US is not a lawyer and may not give legal or immigration advice.
Your bank: many banks notarize free for their customers. Call first.
Your university or employer: many have a notary in an office.
Shipping stores (UPS Store, some FedEx locations) and some libraries: usually a small fee.
Online notary (remote video): allowed in many states, but not every office accepts it. This differs by state; see In your state below.
Fees are set or capped by each state.
Do not sign the document before you go. You must sign in front of the notary.
Bring a valid photo ID: passport, state ID or driver's license. The name must match the document.
Bring every page of the document, filled in except the signature.
If others must sign, they must come too, with their ID.
Check the stamp and date before you leave.
Documents for your home country
Your home country may also need an apostille on top of the notary stamp, or a stamp from your embassy. See Certified translations and apostilles.
A name on the document that does not match your ID.
Asking the notary which form to use for immigration. Ask a lawyer or accredited representative instead.