There a at least two starting points for starting a business, you can have "a great idea” , or "an ideal” you try to fulfill.

For defining your (core) business both your idea and definition of your mission are good starting points. From a well defined mission, great ideas for products and services can be substracted.

So what is that mission, the ultimate goal? It’s the dream the organization (you) pursues. It’s the fundamental need that the organization fulfills in society (opposed by the idea that fullfills a specific need). A mission is timeless. It’s valid now and in the future.

You should ask yourself, what would I like to achieve with my organization? What is the highest reachable goal I can see becoming reality?
Why does the organization exist? What does it stand for? What’s the ultimate goal? For whom do we exist?  What fundamental need does it supply in?

If you already have your idea for a great product and want to define a mission for your company, you can ask yourself, why is this a great idea, what need fundamental need does it fulfill?

An example is Sony, in the eighties they made the walkman which was based on the need of the public to take music everywhere. It has a mission statement more or less like this: "To experience the joy of advancing and applying technology for the benefit of the public." They were forerunners in making technology portable.

The number one bookstore on the internet, amazon.com, seeks to be the world’s most customer-centric company, where customers can find and discover anything they may want to buy online at a great price. They also sell books :-)

Or Coca cola: To refresh the world in body, mind and spirit. To inspire moments of optimism through our brands and our actions. To create value and make a difference everywhere we engage.