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As an American, you should already have a Social Security Number, which is what was used to open your bank account. As a citizen, you do not need anything special, other than maybe a local business tax license, in order to do business here. However, you still should be filing US tax returns each year that you meet the filing requirement, whether it is foreign income or not. You would receive foreign tax credits for any foreign taxes you paid to offset any US taxes.
You should surely start filing now. You would also have a filing requirement for all past years you made certain amounts of income. If you want to be totally legit, you may only consider filing the last seven years, although I've seen some people file a late return only when asked by the IRS for a return.
No Income= No filing requirement. Just ensure you file in the year you start your business.
You can use your SSN, or apply for a FEIN at the following link. Thank you!