how do i make a compatible mobile application/mobile website
Your Question is a bit unclear.
however. if you want to make an Application such as a game or maybe a spread sheet application that runs well on mobiles devices and on your web browser I suggest you learn to program in Javascript/HTML5/CSS for front end browser development and a back-end server language like PHP .
You can make a wide range of web based applications that work across a wide range of devices such as mobile phones(iPhone, etc), tablets(Android. iPad), and desktop(Firefox, IE,Chrome,etc.).
You can create mobile friendly website by using media queries. Such website will displayed correctly on the mobile device. Here is good guide for you https://steelkiwi.com/blog/practical-advice-designing-websites-mobile-devices/ “Practical advice on designing websites for mobile devices”.
