You can go to a police station. A police officer will draw up a record of your statements.
You can also write directly or drop off your letter at the Public Prosecutor's Office. Your complaint must be signed and must describe the facts clearly and completely, with all the important elements (including date and place of the facts, context, names of the persons involved, possible witnesses, medical certificates, etc.). The legal qualification of the facts is not necessary, but it can be mentioned.
Your complaint must be filed within 3 months.