When you are signed in, select "new post" at the top right of the screen, a new post will come up and you can type in your text. The toolbar above your text lets you add images and links, or change the text, font, use italics, etc. To embed video, just copy the embedding code from a Youtube video (it's given to you along side the video info) and paste it in.
When done, click "publish post." It is really easy - and if you get it wrong, you can always edit the post - just click on the little pencil you see underneath the published post to edit.