If I am a beginner ( no coding experience ) where would I start if I wanted to build a social media app like Tumblr ?

I believe Tumblr was built using PHP, but you may want to start with the front end development track first and make your way up once you have the fundamentals down (HTML, CSS, & JS). Good luck!

If you want to make a web application then Abdul's answer is sufficient. If you want to make a mobile app, I would look at the iOS Development with Swift 2.0 if you want to develop for iPhones or the Java & Android Development track if you want to development for mobile devices using Android. Regardless of the path you choose, Treehouse will just provide you with the basic fundamentals you need. There is no comprehensive course that will teach you everything you need to replicate something like Tumblr. After you finish the content on Treehouse, you will have to problem solve and build the skill set for the specific features you need.

Depends on what platform you want to build. If you want to build it on the web, you better learn front-end and back-end web development. There is a front-end development track on Treehouse and there are also many back-end programming language courses such as PHP, Ruby, Node.js, Python, etc. If you want to build it on an Apple device, you better learn Swift or Objective-C. There are tracks for that too. However, there isn't a course that teaches you how to build an app on Mac. All of them are for iOS development. If you want to build it on an Android device, then there is also a track for that, but I recommend you to learn Java first, which has a track at Treehouse too. If you don't know where to start, I will recommend either iOS Swift development or front-end development. They are all quite easy to learn and very useful.