So you have an app idea and you are ready to build it. Hold up, that's not the next step.
First, do a wireframe mockup of your idea. Find someone in a marketplace (upwork.com or something like it) to make a useable wireframe. Schedule some meetings with your potential customers but don't use family and friends, put the time in to finding your customers. Have those customers click through your wireframe. Get their feedback. Ask them questions as they do it. Write down their answers. Decide whether you are solving one of their problems or if they are in awe of your intelligence because you are building an app. Decide what you want to spend to build a working prototype for iOS. Regardless what you hear, hold off on Android until you know if the idea is worth the trouble. If you decide to build it. Great! Execution matters here. If your budget is $800 (no idea what your budget is) make sure that you write all of the requirements for how the app should function: Click this, make this happen and if this, do that thing, or that thing if this. Consider how you'd communicate to customers and drive engagement (notifications, badges, etc), Analytics to know what's happening (crashes, installs, engagement to your app store landing page) 500,000 people visit your app page and 3 installs? You should know that. Lastly, ad supported apps with less than 100,000 installs aren't making a lot of money so think about how you monetize it.
Then be prepared to spend a crazy amount of time understanding how to make money that isn't a passive income of $50 a month. I built an app in my niche and the damn thing has done very well. People call to partner on a regular basis, which is great. I got lucky but it was because I worked on a fairly successful web app in my niche for 5 years before we released our first app.
I sound like an asshole but I don't mean to, it's hard, and I learn something new everyday, but it's fun and I make a little money. Leave a comment if I can help you in any way.