Great! Now, ask yourself this basic question - is it going to solve a genuine problem? Mind you, your perception of a unique idea might not match with the ground realities of the market. So, if your idea solves a problem, then the first thing you need to know is that you should learn about the problem in such detail that there is no chance of missing anything when it comes to offering a solution. It's only when you know the problem in and out that you will find the solution more effectively. Coming to your question about where to start and what to do to get your startup going, first, you start working on documenting the project plan.
It should include everything that you have visualized or thought through to identify the problem and come up with the solution in the first place. It should clearly identify the user demography, geography, and income potential among other attributes that one sees in a market. Next, build a good website because, without it, your startup just won't start...I can't put it more simply. Depending on what you're offering, your website will have the functionalities accordingly and do get a domain-specific email server integrated on the website.
Initially, this is the basic minimum investment that you'll have to make on your own and if we go by funding norms that have been and to a large extent and still is, in existence, the funding bodies will bankroll your startup only after you've run it successfully for at least three years.
Although the government keeps saying that entrepreneurs just have to approach the Startup India ecosystem, the truth is that very few VC (Venture Capital) funds or Angel Investors would be ready to put their money on an entirely untested idea. For your idea to attract funding from scratch, it has to be irresistible but you won't know till you have approached any funding agency.