As comments have noted, Markdown is a markup language like HTML, so its not easy to do WYSIWYG.
There are probably two approaches to getting this kind of functionality.
Server side
Post your markdown text to a server and get the server to return HTML. I've done this a lot in Python, but for Ruby this SO answer suggests maruku. The disadvantage of this approach is that it requires a server request every time you want to view the preview, and then some way to integrate that back into your UI without disrupting the user. Stackoverflow uses this approach with the WMD editor and some C# on the server.
Client side
Another option is to use a client side library to take your markdown "code" and generate html which you can then integrate into your page using javascript. An example of this is dillinger. This uses a textarea and something like showdown to render the HTML in the browser. I'm taking this approach one step further in a WYSIWYG Markdown editor I'm developing called demarcate.js
Or... ditch Markdown
The comments have already suggested alternative WYSIWYG editors... most of these are not markdown editors but rich text editors which produce HTML at the end. TinyMCE and wysihtml5 are good options and I've also used CKeditor before with some success - it has a cool "in-place" feature in the latest version which inspired my Markdown editor above.