Anis,
You would need to run a camera node, such as usb_cam
Then run this for the camera
rosrun usb_cam usb_cam_node
and this for dnn_detect
roslaunch dnn_detect dnn_detect.launch camera:=/usb_cam image:=image_raw
and look at the results with
rqt_image_view
If you have problems, please open an issue
Jim