There are accessory lenses that fit in front of the regular camera lens.
Usually they screw into the filter threads, some have other means of
attaching to the camera. These lenses are fairly inexpensive, but they are
not the best quality. I don't think "fisheye" lenses are available in this
type, I've only seen lenses that were slightly wide angle or telephoto.
You don't say what type camera you are using. If it is an interchangeable
lens camera, you will get the best results using an actual fisheye lens
rather than an attachment. But don't let anyone stop you from experimenting!