The first thing you'll need to do is upload your video onto a website. I upload all my videos onto my facebook account, then make sure that they are 'public' so that they can be viewed by all. You don't have to use facebook, there are other websites too, but that's just an idea.Then just above the box where you type your message in a post, you'll see little icons across the top such as B (bold) I (Italic) and U (Underline). If you look along those icons you'll see one that looks like a film clip. Click on that icon and then paste a link to your video into the pop up box that appears. Click upload/ok or whatever it says, then your video will appear in your post.