How do you start a big project?

I've got an idea for something I want to do for my family. That's it. That's all there is to it. I want to do a web app that can tell my family who is related to whom, how we are related (I have a really huge family), and allow us to share important announcements.

It's got a genealogy component; it's got a small social component.

My career so far has been 7 years of database guy, and 3 years of c# and python.

I've always worked on other people's stuff. Sure I have toy projects here and there.

My question is this: how do you start a new thing from scratch?

Everything is so important. There's login. There's security. There's the front end and how it looks, there's the backend. What's the best database structure. There is everything.

Where do you start? How do you get started?

I think I have all the skills, and I think I can do this. But I don't know where to start.