Carla:
This is a big topic!
If you're looking for the same page (same URL), that's responsive web design. Responsive web pages take the same code and then changes the display/size/order based on the device's width. It requires a shift in thinking about the design, because everything is fluid with percentage widths instead of fixed widths.
Unfortunately, the Marketo "templates" don't work quite like how you're thinking. Instead of just having a "template" only apply styling changes, it changes the entire structure of the code—it isn't quite as easy as applying a separate template to what you've already made and just having it work seamlessly.
The normal Marketo way (which is NOT responsive web design) to do what you want is to create two pages: one for desktop and one for mobile. Then you create your campaigns specifically for desktop and specifically for mobile. For example, you create AdWords campaigns specifically targeting mobile, then you point the users to your mobile landing page.
With that said, making responsive Marketo landing pages is possible if you put a bit more into the creation process or change your workflow. I made a responsive Marketo landing page template that you can download with documentation, examples, and instructions.
Also, Marketo is supposedly going to have responsive landing pages as native functionality sometime slated for the first half of 2014. There are some
apps on LaunchPoint which make responsive landing pages easier, although I've never used any of them. There are also some Marketo-certified
agencies on LaunchPoint which can make responsive landing pages, among other services.
Hope that helps as a place to start!
Best,
Edward UnthankMarketing Operations Specialist
Yesler