When you go to send a text, in the "To:" box, just start typing in the email address for whoever you want to send it to.
Next, you need to attach the video. I'm using Handcent, I can't access the default text app because it'll delete all of my text messages. So I'll explain how to do it with Handcent. Click the Menu button, click Attach, it will give you a long list of items, scroll to the bottom and click Videos, it gives me three options which are ASTRO, Files, and Gallery. Just pick whichever one is relevant to you, depending on where the video is saved.