You should start to learn:
1. PHP (this is the programming language they use)
2. MYSQL - learn how to work with databases, how to "design" them (table relations, indexes and so on) how to insert, store, retrieve data with PHP and so on.
3. JAVASCRIPT - you will need it for frontend or backend "interractions" with users, you may want to learn a well-known javascript framework like jQuery
(they use their own "foundry" JS framework), there are others out there like - backbone js, angular js ... and so on
4. JOOMLA - we use the Joomla "ecosystem", framework and CMS - learn it's inner soul, how it works, how it's built, how to develop custom components, modules and plugins, learn the MVC pattern .... and everything related to Joomla Development.
5. HTML/CSS/PHOTOSHOP - you need it to design your apps, you may want to learn some available design "frameworks" like Bootstrap (learn it's classes, how it works, grid system and so on)
..... or if you are ambitious you can build your own CMS using some other frameworks found out there (Cake PHP, Zend, Smyfony2) or build it from scratch (wich is pointless)
In other words you must become:
1. A programmer (php,phyton and so on)
2. A database architect (mysql .. or any other DB technology of your choice)
3. A frontend / backend developer (javascript, javascript frameworks)
4. A joomla developer (brain surgery is different than heart surgery although both of them are "surgeries"
)
5. A designer (Html / Css / Photoshop .... you want your apps to look good, right ? )
+
6. All of the above are stored on a server, so you may want to learn some Unix and all server-side technologies, scalability, clustering and so on.
So, if that's too much for you, start with the most easy part: Html & Css
. Really, it's not easy because you must learn the basics then advanced stuff, then all kind of frameworks and new technologies. It takes time, many years !