What next after getting that zillion dollar idea?

Okay .. So I got an idea few weeks back which I strongly believe in. I know I can make that and it will really gonna go big. I have the technical and other required knowledge, skills to work on it.

The problem is with me I don't know how to make an initial approach towards the idea. Where to start ? What to take up first ? Whom to ask for guidance ? Should I approach to an investor and get some seed fund so that I can completely concentrate on my idea leaving my well paid job ? These questions are going on my head every time I sit to work on my idea. I'm totally confused ....

Sometimes I feel lets start making an web interface for and then I complete some module. In next session I feel starting with mobile app would be great then again I do some R&D or build some sample screens and then done. Next session same story

I seriously need some mentoring and I know the readers out here can help me out with this problem. I'm afraid getting lost

Btw, This is my first post on Hacker news and my big Hi to everyone other there... This community keeps the entrepreneur inside me inspired