If you are a witness to a crime, the police may ask you questions about what happened.  You do not have to give a statement to the police.  You should be polite and identify yourself, but you can decline to make a statement.

You may also be ordered to come to court to testify by a written order called a subpoena.  If you receive a subpoena, you must come to court at the time and place identified in the subpoena.  In court, you will be asked to swear or affirm that what you say is true.  This is also called testifying under oath.  It is important to know that if you testify under oath, you must not lie.  Making a false sworn statement that you do not believe to be true is a crime called perjury, a class B felony.