First you will need ros-melodic-full installed in your computer.

Now:

sudo apt install ros-melodic-ar-track-alvar usb-cam

The first thing that we need to try is to get images from your computer and send them through ROS nodes. For this we can use the test provided by usb_cam.

roslaunch usb_cam usb_cam-test.launch

if you have multiple video devices you can choose which one to use with the video_device option. THe default value is /dev/video0. Here you can find an example of how to run the test for another video device:

roslaunch usb_cam usb_cam-test.launch video_device:=/dev/video1

For more information about usb_cam node options visit This ROS wiki page