Before you start
So let's say you have this great idea for a new app. That, of course, is going to be a real game changer! But before you start your journey to becoming an app proud owner and conquer the world (maybe?), please there are a few things (IMHO) worth taking into account.
As for today, there are 3.3M apps in Google Play Store and 2.3M apps in App Store. American scientists (sic!) have proven that most of them are inferior, but there are a few very good apps in the stores, too. My advice is to check how the creator has resolved the problems with UX and UI, and what makes these apps different from the rest.
iOS and Android users are not the same users (yes, I know this is a pain but if you want to create comfortable user experience you need to design your app according to iOS and Android guidelines separately).
All the ideas you can easily check with a wide range of prototyping tools. Just try to create the first prototype of your app with a few functionalities and show it to your colleagues. You can discuss your ideas using your prototype. From my perspective, the flow is the key to engage your users. Just try to recreate the user flow actions in your app before a development period, and then test ideas with your app users.
So .... Prototype, prototype, prototype (and you will thank me later)
If you have found this paragraph interesting, you should also check a great book "Lean Startup" by Eric Ries (don't know the guy personally, but the book is cool).
Your app will be as good as the time you will invest in it.