Assuming you've already tested the validity of your idea, and you have a small team and little budget, here's how I've approached that challenge in the past:

Plan. Plan until there's zero mystery left.

Abraham Lincoln has a famous quote about planning, which I'll paraphrase: give me 6 hours to cut down a tree, and I'll spend the first 4 sharpening my ax.

I would start with a simple flow. From the home screen, where can the user go? What do they tap to get there? Is there a loading screen?

Plan the entire sequence, which gives you an idea of every view, transition, and UI component needed to make the app run. This avoids surprises later in the process.

I usually use a mind map for this, because it's quick and simple to build using an online tool.

Next, make a "paper prototype": with no design, start laying out each view and transition. Focus on getting the navigation into the right places, making sure next actions are clear, and ensuring that placement is consistent throughout the experience.

You're not looking to make it pretty here. Use a shitty font (like Comic Sans) and boxes with Xs in them to visually enforce that you're NOT designing; you're laying out.

After the wires are ready, plug them together in a user flow. A service like InVision is good for this, because you can make your wires clickable.

Use this to do basic user testing. Can a user find their way to each action they need to take? Do they have trouble using the UI? Where do they get stuck?

Iterate through wires until you're confident the UI is good and no necessary actions are missing.

Design comes next. You're all set with function, so you can focus on form now.

With a design completed, develop the software to power it all.

If your planning was effective enough, you'll feel like all the other phases were too easy — that's the sign of a very sharp ax.

I'd be happy to help you with planning if you're not sure where to start. I also have an article on planning, if you're looking for a starting place: http://lengstorf.com/effective-project-planning/

Good luck!