The first unofficial step is to develop a specific idea of what exactly your company will do - services, products, etc. - and the first official step will be to write a business plan. Many resources exist to help you write a business plan, but developing the idea behind your company and your business is really up to you.
Start with these steps, then move on to implementing your plan (finding funding, registering with state and federal liscencing organizations, procuring a physical space and so on).
This is good question. It is great that you want to start a company, but you will need a product or service worth people's time and money. If you do not have a great product, then your company will not succeed, no matter how hard you try. So, the first step is to find something worthwhile or improve on something that people need or want. This will be the hardest part.
Second, assuming that you have a product or service, you will need to make a team of people who have different gifts - administration, marketing, finance, etc. Also you will need to determine what is the most cost effective way to sell your goods or services.
Third, you will have to think of all the logistics behind delivering your product of service. Finally, there should be a constant review of how the company is doing to ensure that you are working effectively and well.