If you're operating a sole proprietorship, then you can simply use your SSN for tax purposes. Since your business's earnings will come through your personal US tax return, this is the only federal tax ID number you'll need. If you operate any type of business that isn't a sole proprietorship — like an LLC, partnership, or corporation — then you'll need to have an EIN number, even if your company doesn't have any employees or payroll. You will also need to look into obtaining a state tax ID number on top of your federal ID number to legally report state-level information, like sales tax.