The intent of a startup is to develop a product that people will buy from you. It should be a product that isn't already on the market. If you are thinking another facebook/google/youtube/dropbox/trailers/angrybirds/office productivity clone of existing products: Stop, go back to your desk at work and keep churning out code for someone else.
You need a unique/newish idea:
If you have an idea you think is actually new and useful. Do some googling on the idea to see if anybody has considered it (or similar overlapping ideas) and the response to it. Do some basic patent research to see if anyone has staked ground on your idea. Do some informal research to see if people are interested in (and would pay money for) your idea. If you feel you have to to mask the purpose of the research, go for it (it just takes more time). Surveys can be written to show interest in one item while simultaneously drawing out information on a not so obvious second. Now you have an idea that should be relatively unique, patent free and people want (even if they don't know it or can't articulate it). If you can't answer these questions, stop now.
Get a lawyer
You will need to incorporate at some point and a business lawyer will be able to point your in the right direction on the pros and cons of different company types. Also plan with the lawyer on your future end goal (expand or buyout) so your company can be structured properly for future action.
You need manpower, budget and a business plan:
How many man hours will it take to have a demo model (no matter how crude)? How many man hours will it take to bring it to market? What is your time frame to bring it to market? Do you need to quit your current job to work on it? Do you need to hire people in order to meet your deadline? How much runway do you estimate you need in order to get off the ground with a saleable product? This is the basis of your development and launch budget. Your business plan is your estimate on keeping the company operating: what are your liabilities, assets, income and burn rate of reverses?
Where is your budget coming from?
If its just yourself you have to go back to a day job when you bank account is empty. If you are employing other people you need to continuously be out there pitching for seed money (which takes away from dev time). So having a working demo that you can pitch is needed. Have your pitch smooth and bullet proof and be ready to field all sorts of outlandish questions, be told "we'll think about it" and then ignored, and to be told NO many many times.
What is your end goal?
Do you want to develop into a larger company or be bought out for a pay day? Do some research on your Phase II goal. It doesn't need to be now, but you don't want to be surprised when you turn around and realize that you are suddenly employing 35 people and you have a large company knocking on your door with an offer and you don't know what to do with it.