If you have a flash animation that you want to display in your HTML page, then you need to put HTML code in to do so (search on the "object" tag). If you are asking how to create Flash content, then you would need to use the Flash program, or perhaps an online Flash content-generator. Also, if you're specifically going for a Flash video, you need a player as well. FlowPlayer is a nice free one out there (instead of having to code your own). Here is a good
general purpose article that touches on all of these items.