Ok using HTML5/CSS3/JS creating mobile websites is easy. But what if you want to go beyond and create app.
Well then you need to go for some third party support.
If you are a company/have money to spend, you can try out something like http://www.orubase.com/ from Syncfusion. This allows you to take your ASP.NET MVC skills and create native apps for multiple platforms. Also it provides native JS bridges which means you can also use device features like camera, GPS etc. in you application. Your HTML5/ASP.NET MVC code gets wrapped into a native application. Also since Syncfusion is a UI components company, Orubase includes a number of UI components that you can use to build your apps.
If you are a hobbyist, then you can always go for Cordova (aka Phonegap). This does mostly what Orubase does, except it take pure HTML5/JS/CSS3 and not ASP.NET MVC. Cordova is an Apache open source now and you can
You will still need to learn APIs specific to these platforms, but these are usually easy to learn JS libraries in most cases.