Ask HN: How do you get started creating a webapp?

I have a few webapps I'd like to make, but every time I start, the interfaces I create look so juvenile and plain (in a bad way). I look at websites like AirBNB or Evernote or Facebook and they look so clean and sleek. And I can find plenty of articles online about webapp design patterns (grouping like things, using colors in ways that people are used to, etc.), but how does one actually start? Even in terms of just putting a navigation menu next to a page of content, it always just looks so poor.

Any advice?

Thanks so much!