Assault is committed when a person engages in conduct that puts another person in a situation where he can reasonably fear they will receive a battery. A battery is the act of physically harming someone else. It is a threat of bodily harm. Assault is a crime, and you can press charges against someone if you are assaulted.
Go to your local police department and tell the person at the desk you want to file a complaint.
Tell the police what happened. Fill out the form the officer gives you, or give him the information he needs if he fills it out himself. It will include your name and address, if you know your assailant, his name and address, a description of the incident including location, date and time. If you don't know the assailant, the police will request a description.
Take your copy of the police report for your records.
If you can identify the assailant, the police will take the complaint to the prosecutor's office who will decide if there is enough evidence to prosecute. If they think there is, they will get an arrest warrant from a judge. Once you've decided to press charges, the police will investigate the crime.
If you fear retribution by the assailant, after you file the complaint, you may inquire about a peace bond or order of protection. If this is a concern, consult your local jurisdiction before filing the complaint.