Let’s say I start a company, of which I am the sole owner. I sell it tomorrow to GE for N dollars, where N is a very large number.
How much of N do I have to hand over to the federal government in April 2009?
There is no way to answer that question in generalities at all. That is what CPA’s are for. That stuff gets complicated. Owning a company means that you get to depreciate assets over years and move money around in all kinds of complex ways. It takes a corporate CPA and maybe even tax attorneys to sort it all out. The IRS can’t give you a number. They can just give you hell if you defraud them.
How long have you owned said company?
In general, your net profit is taxable. Total amount put in <amount already deducted>= Basis. Price RCVD <Basis> = Profit. This is very general.
No idea. But can I be your friend? 
And a good CPA or tax attorney will help you before the sale. Tax planning is something you do before you incur the income or gains on which you will be taxed. You don’t go to a CPA and say, “I sold my company for $10 million, what do I owe?” You go before the sale and say, “I’m negotiating to sell my company for $10 million. How do we structure the deal to minimize the impact of taxes?”
As others have said, there’s no real way to answer that. I would add that if it’s sold for stock rather than actual dollars, the answer could very well be nada. You would just wind up holding GE stock, and would not pay taxes until you chose to sell the shares, which you might have at a basis of nearly zero.