For images, as long as the image has a valid file extension (.png, .jpg, .gif, .bmp, and others) all you have to do is put these tags (without the spaces) around it: [ img ] [ /img ]
There's also a button that will put these tags in your post for you. It's located on the middle row of all that stuff above where you type your post (between the smileys and the bold/italics/etc stuff), and it looks like this:
.
Videos are a bit different. Videos from Youtube have built-in, automatic embedding, meaning you simply need to post a Youtube link and it
should display the video in the post instead of a link. The shortened Youtube URLs may not work, I dunno since I haven't tried.
Videos from other sites may not be automatically embedded, so you'll just have to post them as a link; thankfully, the site recognizes when you post a URL and makes it clickable on its own.
Let me know if any of that wasn't clear.