I know this might seem weird but I don't know how to start building an app.

A good idea is to start away from the code, take out a pen and paper, or a note taking app, or use https://trello.com/, and write down what your idea is. Then come up with the really high level concepts of the app are, say you're making a weather app, you might need to get the users location, get the info, and display the results to the user. Then dig deeper into those, what is required to get the users location? Where are you going to get the info from? Record these steps as well. Then, start breaking down these into smaller tasks, and begin to think about the order of implementation -- you can't get weather data until you have a location, so this obviously comes first. Also note that you shouldn't be worrying about HOW as much as WHAT. You'll then have two important things, 1. a list of bite size issues/features you can tackle, and 2. a general idea of the order you need to tackle. Now that you've got that setup, whenever you face that programmers block, for lack of a better term, go to that list, and pull off a task, and begin writing the code for it. Realize that this is a living, breathing process, your specifications will probably change, you'll probably rethink a lot of the details, but by having a roadmap, you'll be able to always have a path forward.