I'm using wyswim editor markitup from here: http://markitup.jaysalvat.com/
The problem is I need to press a button to preview the content.
I want to add live preview as I type in the content. Something like what stackoverflow does.
This is something you will want to do on the client-side and not on the server (as mentioned above). Also, WYSIWYM stands for What You See is What You Mean - not at all what MarkItUp! is - http://en.wikipedia.org/wiki/WYSIWYM.
You might have more luck implementing an editor with this feature baked in - though I'm sure it wouldn't be too hard to add live preview yo MarkItUp! - such as EpicEditor (click to edit in full screen) http://epiceditor.com/