First, you'll need to have a Premium Photo or Premium Plus ad in order to embed the video in the ad itself. You can, however, place a link to your video in the comments area on any ad type.
Once you have your video file on your computer, it can be uploaded to YouTube directly.
Here's a link to the Youtube help area which contains much information about
video file formatting and uploading:
https://support.google.com/youtube/topic/16547?hl=en&ref_topic=2676319
Once uploaded, you will then be able to obtain special HTML code from the Sharing Tab for your video at Youtube. You paste this into the text box on our website when you edit or place your ad. You don't need to upload the video initially…you can login later to do it.