This question is pretty vague but I’ll give it a shot. A few options:
1) Check out WPtouch – http://www.wptouch.com/ – This plugin gives you the ability to override certain aspects of your primary theme with mobile friendly templates instead.
2) This is not ideal but you could use some basic Javascript to check the screen size of the current device and manually redirect any “mobile” screen sizes to a new page.
3) Finally, if #1 is overkill and #2 is too basic you’ll need to go the custom code route. Specifically you need a device detection script or library. This will allow PHP to detect when you are on mobile vs. other devices. Once you know what kind of device you are dealing with you can code some logic into your theme that displays the desired template or content based on that device.