I get this question all the time and it is such a board question it says loudly the person asking it is not ready to launch any company yet.
I spent 5-10 years preparing myself to launch a company by reading hundreds of books, being a manager and Vice President, going to SBA and MIT Enterprise Forum meetings and other venues and far more.
The answer is my CEO Boot Camp and years of preparation to understand sales, marketing, operations, finance and product development (your domain/industry). I also created a book called The Startup Manual with a CD-ROM of digital tools to walk you through the entire process of design, launch and grow.
My products are avaialble at www.StartupPlanet.com but I would strongly recommend an experience CEO Coach and/or advisory board.
All that said basic service businesses are simple to run, while technology businesses that are meant to scale are far more complex and require lots of experience.
Some general things you need:
1. Clear target market or ideal customer profile
2. Clear value proposition and messaging that resonates with target customer - ideally a customer that understand the need and value, not one that needs lots of education, which is expensive in sales process.
3. Business model design (an art very few can do well that requires decades of experience)
4. Financial plan and/or capital to start
5. Team that compliments each other. See my Best practice video here.
All of this together and more is called a "Vision" and I have an article on this here.
I hope these resources help answer this broad question. It cannot be answered well in a sinle article by anyone and the only good answer is to point you to many resources and recommend study, practice and mentorship.
Anyone that needs to ask this question is not yet ready and should likely work for someone else for a few years before they lose their shirt trying to launch a company.