I’ve been developing apps from a few years now and there are some typical things at the beggining (when you are thinking and designing the app) that you should do if you do not want to be overwhelmed.

1. First at all, do a meeting with your customer, see what does he need and what does he really want. Believe it or not even customers don’t know exactly what they want.
2. Discuss with your team about your possibilities, period of times and efforts.
3. Prepare a draft of the app (I use Inkscape for that) where you can explain the interface and the basic user experience.
4. Discuss again with your team. Remember that no matter if you are a great UI/UX specialist or not, sometimes your workmates can offer great ideas (new ways of thinking).
5. Show the strengths of your app to the customer and be open to do some modifications.
6. Finally, think about the budget and time of developing and sign a closed contract with your customer.
7. So important, once the contract have been signed, any extra modifications have to be made with a new contract if you don’t want to get mad or/and unproductive.

Here you can see an initial basic draft. Do not extend yourself too much doing it wonderful and perfect because you will have to change it a lot (it has to be just the first idea), remember you will have more meetings with your team and customers.

Please, do not make the mistake of starting with the code!