Incorporating an online business
Your tax liability would depend greatly on the state that you, as the owner of that company, claim as your domicile, and the type of entity you choose.
If you choose LLC or S-Corp as your business vehicle the company profit (or loss) would pass through to your personal tax return and you will pay the federal and state taxes according to your personal situation. C-Corporations are taxed on corporate level first and then the dividends are paid to the shareholders and taxed at the shareholder level. In that case Wyoming, Nevada and a few of other states would work as they have no state income tax, however keep in mind the federal income tax that does not depend on the state.
As far as nexus goes, selling services or products online does not by itself create nexus in the states where you sell, so in your case no foreign qualification is required in other states. With that being said, you might be in a position to having to foreign qualify in the state where you run your business from, as your physical presence as the manager of the company and possible home office connection might warrant the need to foreign qualify.
I would suggest you to consult a tax specialist, licensed in your state, for all tax related questions, and legal specialis licensed in your state for the question of whether or not your business (if registered in another state like one of the incorporation heavens) is required to foreign qualify in your state.