Building your application
Whether it is standard HTML or dynamic code like PHP or ASP.NET, you need a good editor to put it all together. I have used a number of editors over the years, and the one I like and use daily is WebMatrix. The latest version is a free download and has everything you need to build prototypes and the final product. In addition, there are a variety of templates available to quickly build an application.
I know Microsoft doesn’t have the best reputation in the web development community, so if you choose to explore other options, you might start with Dreamweaver, Brackets, or a cloud-based tool such as Cloud9.
A good foundation
There are freely available frameworks that provide the foundation for your own application; these frameworks utilize the latest techniques for full-featured web applications. My favorite is Bootstrap, and the latest version fully embraces responsive design and the mobile platform — the motto of Bootstrap 3 is mobile first.
Bootstrap allows you to build powerful interfaces that conform or respond to the user platform. It is a simple concept but, like anything, it takes time and patience to get acclimated with the framework (nothing beats diving into the code).
There are plenty of other options available, such as Foundation. The key is to utilize a framework that conforms to standards and allows you to concentrate on application design as opposed to the nitty gritty details of concepts like responsive design.
Plenty of options