Your dermatologist is looking for things on your skin. He's a medical doctor, seen everything, and is interested in looking for diseases of the skin. He's not interested in oogling your body. I'm a guy, and I had a female doctor for years. She was a good doctor, and being a woman was irrelevant to our relationship of doctor/patient. That's how it is.
Don't be scared about possibly finding a skin cancer. They're highly treatable as long as they are discovered early, and being as young as you are, it's good you are getting it checked out.
You'll be given a "garment" that you will wear after you've taken off your clothing. The dermatologist will look at one area, which you will expose, and then go on to additional areas. He will check the particular area that brought you to the office, and then also check other areas that are common problem sites. You're lucky to have people care enough about your health to make sure you're getting checked out.