Information about preparing a witness statement can be found on the Citizens Advice website. The witness statement should be typed, in numbered paragraphs, and with pages numbered. It will form your principle evidence to the Tribunal, and will be given under oath. You will be asked questions on it at the hearing by the Respondent or their representative. Therefore you should make sure that it is an accurate and complete account of the events that took place.
The Employment Tribunal will order you to exchange witness statements with the Respondent; this is usually done simultaneously by email.