I have built a few startups in the past, and am currently building 12 startups in 12 months at http://blog.12startupsin12months.in/ . Like the @nfriedly mentioned. You really have 2 questions. I always ask people to get their product validated with at least 20 customers. For this you probably just need a simple landing page asking for an email. Make sure you are building something that provides value.
The next part is actually building the app. I am a developer so my advice may be a bit biased. But, I would ask you to learn a front end framework. If you don't know an JS frameworks yet, I'd ask you to take a look at Elm. Once you have the first version out, and grows beyond your capabilities you can hire folks to work on it.
Well, what is the difference between a hobby project and a startup? Is a startup something that brings in money? If so, I am definitely planning to monetize all the 12 startups. Does a startup need someone who works on it full time? If so, I am working on these things full time. These products may be small but they are real products which will have real customers.
Yeah, The first app that I am building has a lot of competitors. So, it is not really a question of whether the market is there. I definitely need the confidence If my startups are going to be successful :)
The next part is actually building the app. I am a developer so my advice may be a bit biased. But, I would ask you to learn a front end framework. If you don't know an JS frameworks yet, I'd ask you to take a look at Elm. Once you have the first version out, and grows beyond your capabilities you can hire folks to work on it.