Do not shower, bathe, douche, eat, drink, smoke, wash your hands or brush your teeth until after you have had a medical examination. Remember to take a change of clothes to the hospital, as they will collect the ones you're wearing as evidence. If you do change, save all clothing that you were wearing at the time of the assault and bring them and any other potential evidence to the medical exam. Place each item of clothing in a separate paper bag. Do not use plastic bags. Do not clean, straighten up, disturb, or remove anything from the area where the assault occurred.
A forensic examination is one way to preserve evidence, but it is not the only way. You can easily take some important steps to preserve evidence by saving all text messages, emails, social media postings (taking screenshots can be helpful) or anything else that might relate to the assault, or that might be helpful later in reconstructing a timeline of events. Write down the names of people who might have seen you immediately before or after, as it's easy to forget names or locations. Even if you do not want to participate in the investigative process now, you might later change your mind, so it's helpful to preserve as much information as possible.