Most people accused with having committed a crime consider hiring a criminal defense attorney after being arrested or cited with a crime.  An arrest is generally deemed to take place when the accused is taken into custody by the police (or another government agency).

A person is usually arrested for one of three reasons: the police feel they have probable cause to believe that the accused committed the crime; the prosecutor filed an Information or obtained a Grand Jury Indictment formally charging the accused based on materials investigated by the police and presented to the prosecutor's office; or a judge signed a capias (an arrest warrant).  Most arrests occur because the police feel they have probable cause to believe that the accused committed the crime.

In a misdemeanor case (punishable by no more than one year in jail), at the time of the arrest, if the police have enough identifying information concerning the accused, then, in many jurisdictions, the police may cite the accused with a crime and release the accused with a Notice to Appear to court.  Many people falsely believe that the Notice to Appear is just a civil infraction (like a traffic ticket).   A better way to look at a Notice to Appear is similar to being released on your own recognizance at the scene by the police instead of being taking into custody and brought in-front of a magistrate for a first appearance to determine bail.

In a felony case (punishable by more than one year in prison) or in a misdemeanor case where the police did not issue a Notice to Appear, the accused is brought in-front of a magistrate without unnecessary delay (usually within 24 hours of the arrest) for a first appearance to determine bail.  At the magistrate hearing, the criminal defense attorney will attempt to show the magistrate judge that the accused is not a flight risk and is not a danger to the community.  The magistrate judge will determine the conditions of release (if any).  If the accused is able to post bond, then the bond will hold the accused person's place in the jail as long as the accused does not violate any conditions of the bond.

Since most arrests occur because the police feel they have probable cause to believe that the accused committed the crime, the prosecutor has not yet made a decision as to what formal charges will be filed with the court.  If the accused has reason to believe that the accused should not be formally charged with the crime(s) which the accused was arrested, then the accused has a small "window of opportunity" to have a criminal defense attorney present such reasons to the prosecutor prior to the prosecutor making a filing decision.  This time period is critical in many cases.