X-rays are the most common and widely available diagnostic imaging technique. Even if you also need more sophisticated tests, you will probably get an X-ray first.
The part of your body being pictured is positioned between the X-ray machine and photographic film. You have to hold still while the machine briefly sends electromagnetic waves (radiation) through your body, exposing the film to reflect your internal structure. The level of radiation exposure from X-rays is not harmful, but your doctor will take special precautions if you are pregnant.