Include dates, gestures, comments and your response. Be as specific as possible. Write down everything, whether you think it is important or not. It may be very important later on. Keep any gifts, letters, pictures, paraphernalia, regardless of how distasteful it may be.
Physical evidence can be important.
If you find it necessary to resign, please inform the employer in writing that you are being forced to leave your job due to sexual harassment. Keep a copy of this letter. If you leave your job or are fired you may need to contact them to substantiate your complaint. Obtain a copy of your personnel records. You can get all personnel records that exist, not just the ones that your employer keeps in what it considers its personnel file.