They are assigned an attorney by the court.
Clarence Gideon had been charged with a burglary in Florida, and the judge said he could not appoint an attorney for Mr. Gideon because the crime of which he was accused was not a capital offense. Gideon claimed he was entitled to be represented by counsel, but the judge did not agree. Gideon was convicted and went to state prison. From there, he appealed to the US Supreme Court in a suit against the Secretary of the Florida Department of Corrections. (By the time the case came before the Supreme Court, that was Louie L. Wainwright, thus "Gideon v. Wainwright.")
The Supreme Court agreed with Mr. Gideon's claim, and since then, all persons, whether in state or federal court, are entitled to the right to counsel and an attorney is appointed if they cannot afford to hire their own.