Prototype, build, ship, rince, repeat!
Wether you go iOS, Android, Web, that doesn't really matter, you don't need a designer to build a great app. All the tools are available there.
You don't need it to be perfect, just to be built.
If you idea is awesome, people will use it. The better the app, the more they'll use it, but for now, like Eric Ries would say, get out of the building!