Clearly, set your objective about your app objective. E.g., Find petrol stations on a highway in a particular state or country. Visualize how your app will look on the screen. Draw a rough diagram of screens or skins. What are the functions and everything in between? Know if your idea is already used. Find out what users are looking for? How to tap the market financially? Chalk out a marketing plan. Marketing isn't the last step in app making. It should be taken along in every step i.e., pre-production, production, and post-production. Storyboarding helps you have a blueprint of app functionality. Similar to storyboarding for movies, the more elaborate storyboard is, the more lucid the process becomes. Prototyping tools let you see and test your app in real time. You can check and modify accordingly. Ask your friends and family to check your app and give feedback. Incorporate their feedback in your modifications. Now after testing the prototype, begin your work on the backend. It is the developer's' side of the application. It includes storage, APIs, setup servers, and databases. App store requires you to get registered as a developer. You don't need to be expert in programming but it requires a fee. Check out the sites for more info. It is the actual creation of the skins or screens that the users will use. You make UI at this stage.

Summary:
Decide what your goal is. Do some paperwork. Research the market. Storyboard. Make a prototype or wireframe. Build the backend. Get registered. Skin/Screen it.