Work at a busy ME office as an autopsy tech and we regularly have students (Nursing, EMS, High school, and College) all come through to observe. For clothing, closed toed shoes and long pants are a must but ask the office what else to wear. You will probably end up wearing scrubs or jeans and t shirt with PPE on top. Know what an autopsy is and the basics of it (Youtube and Google). Please eat food. It doesn't have to be a big breakfast or anything, but something filling. People do faint, but it is because they did not eat the morning of. This is why you eat.
The progression I see in the majority of our observers is the following: A natural repulsion/uneasiness (After all, this isn't an everyday thing for most people), followed by curiosity, and then fascination. A lot of pathologists I have worked for love to teach and revel at the opportunity to show anyone anything. Ask a tech at the beginning where to stand and when is appropriate to ask questions to the doctor. At our office we make sure all observers stay out of the doctor's way while also keeping them engaged. If you are there for a homicide or something of that nature then just be a shadow unless spoken to. The first autopsy I ever saw the pathologist was big on letting me hold and touch things, in their words "The cool part about this job is you get to poke s---." Take advantage of these opportunities. What ever you do: Do not psych yourself out. This is what gets people the most. It's okay to go in a little nervous, but don't hype it up to extreme degree in your head. You will probably notice how lighthearted the staff is. We make jokes all the time during autopsies, and classic rock is always playing. If we were super serious 110% of the time we would hate it. The smell will probably be your biggest issue since it is something you have never experienced before, but you will quickly get used to/over it. Before you go back, or put a mask on, put a mint in your mouth. It will cover the smell a good deal if you are worried.
This has the opportunity to be a really cool thing if you find yourself fascinated by it while your there, and if it is not your thing that's okay too, it's not for everybody. Go in with an open mind and a desire to experience something new. If you like it, try and observe as many cases as you can. You will learn something new every time. PM me if you have more questions.