If you use the Google Webmasters for your webpage then you most probably have seen the Mobile Usability page under the Search Traffic menu. In this subpage Google lists for you CSS, HTML or SEO errors that have to do with the mobile version of your website. One of the most common errors that is listed there is the Content wider than screen error.
When dealing with User Experience and user interfaces you often have to do with prototyping an idea or a concept, before going to the implementation of it. Doing this way you want to get a first impression of what you are about to implement. Prototyping can be divided in two categories:
Low fidelity prototyping: You create sketches, also known as wireframes, of the user interface you want to program. For creating these sketches you use either a pen and a paper or one of the many available software to draw the sketches digitally. This software can either be a professional one (Photoshop) or a more pen & paper like, like Balsamiq.
High fidelity prototyping: You implement a small part of the future application with a limited number of features. The application is testable and you can navigate from the one page to the other. Here the business logic plays a secondary role, the user interface the preliminary. A high fidelity prototype could also be considered a collection of Photoshop designs, each representing a page of your application, that are combined together to build a fake navigation from the page to the other.