Draw a mockup on paper and figure out how you want your app to work. Once it's done, get the simplest case working in real software.

I find that by getting something simple working helps me keep motivation whilst going forward, even if the code is crappy. Conversely, I've started a non-trivial number of webapps by designing a DB schema and an application architecture, each of which left me burnt out before anything of value was even written.