This video on how to embed youtube videos is a bit out of date but the strategy is the same. Mostly you copy the iframe embed code from youtube and paste it in post box here.
1) Use the youtube site to find the video you want. 2) Click the 'Share' button below the video. 3) Click the 'Embed' button next to the link they show you. 4) Copy the iframe code given and paste it into the html of your web page.