I have a WP site. Now wanna create an iPhone app base on the content from my site so that the visitors can read my site easier.
I wanna create one just like the iPhone app by Mashable (the Mashable should be based on WP)
http://itunes.apple.com/us/app/mashable/id356202138?mt=8
Anyone can advise me how should I start?
Or any template of such?
There are two main ways to do this: either you create a complete iPhone application, written in Objective-C, that gets data from your blog via an API. This can be regular RSS feeds, XML-RPC or a completely custom solution. Be prepared for a lot of development in both Objective-C and PHP (because your API should be secure of course).
The other option is to use or develop a mobile-optimized theme, with extra tricks to get more integration with the device, like GavinR hinted. This is much simpler, but won’t offer everything that a custom application can (no offline access, for example).
I would suggest a slight workaround, by making your site mobile compatible with a plugin. This allows all platforms to see your site, and the iPhone version looks much like an app (page transitions, big buttons, etc).
To add some additional flare you can add an iPhone icon so that the user can place a shortcut to your site on his or her home screen:
More info at the Apple Dev Center.
I have created a list of points that you need to follow in order to develop an iOS app for your WordPress blog just like the example app of Mashable that you have given in your question. Check out my answer HERE