Disclaimer: This guide contains how to add videos to your website strictly from YouTube. It's far more easier to utilize YT videos, rather than uploading the video to your site. Here is an article on why you should never upload a video to your website.
How to add a Video to a Page
To add a video to your page, you will need to head over to General Content > Spark Pages > Edit Page (Or add a new one)
Once you are in the Spark Editor, Add a new block,
and then choose Header under the categories.
Then, add one of the header blocks.
Click on the block to edit it.
Collapse the Background options, and under the Background Type, select YouTube,
And paste the YouTube URL.
Once you are done, feel free to Save the changes and exit the page.
How to create a Video Post (add a video to your post)
In your dashboard head over to General Content> Posts > Add New (or edit an existing one)
Make sure your post format is set to Video.
Then, grab the URL of your YouTube video you want to add, and paste it into the editor.