I usually start with a MindMap where I try to write down every bit of the idea.
I then try to set requirements and set the tools/frameworks I want to use.
Next step would be data model and then git started ;)

Mindmaps are great. My friend wrote and founded Mindjet (Mindmamager at the time) while critically ill and confined to a hospital bed. It became a worldwide sensation.

I usually start with mindmaps too until I go way too deep into ideas and complexity, and it just make me give up before even starting.

I know I should keep it simple first, but I write down ideas because they're good, I can't let just skip them or I'll forget them :(
(I have terrible memory)