Hi Sherin--responsive landing pages are a bit tricky in Marketo. You are probably going to need to use some relatively heavy CSS, jQuery, and HTML. You'll need to set the appropriate media queries for desktop, tablet, and mobile device break points. Here are some good articles:
http://edwardunthank.com/blog/how-we-made-a-responsive-marketo-landing-page/
http://edwardunthank.com/blog/how-to-make-responsive-landing-pages-in-marketo/
Importantly, if you use the methodology outlined in those articles, the landing page editor won't be much help to you. Since it doesn't call external CSS or jQuery in preview mode, it won't really give you a good idea of what your page is looking like. Good luck!