As the complainant, you will be protected from retaliation either under whistleblower protections or the terms of the bullying and harassment policy.
You are entitled to have your complaint taken seriously and investigated fully. You should be allowed to provide whatever evidence you feel supports your complaint including documents like emails and texts.
If you are uncomfortable discussing your complaint alone, you can ask to have a support person attend the investigation meeting with you. You do not need to have a lawyer present in most cases.
If you believe that the person investigating is biased, you are entitled to ask for another investigator to be appointed. However, your employer may or may not agree to this request.
If you refuse to participate in the investigation, your complaint may be dismissed without further investigation, or your employer may decide to conduct the investigation based on what has already been raised and contact other possible witnesses even if you are unwilling to be interviewed.