If your site has not been set up by me, I can amend your Joomla website so you can do this.
Ideally videso want to be uploaded to YouTube or Vimeo or something similar: so they can be found in multiple ways and so that you are not using your own bandwidth.
Then you can insert them using a template I have set up for you - using this template means the video will grow/expand according to the users device.
If the video was on Vimeo or YouTube etc, then choose that from Media type.
If you want the video on your own site - not really recommended unless you need people in China to see it - then media type is html5 video and you select the video by finding it / uploading it to the correct folder and selecting it in exactly the same way that you do images.
You can either enter the new URL here from the YouTube embed or just Copy & Paste the video id ie "6UGbsEI06gI"