If you haven't set it up as a LLC or Partnership you need to file it as a disregarded entity in your personal tax return using schedule C. So you would need to amend your tax return to add schedule C. Doesn't matter if you have people working for you or not.
You can't do it separately from your personal 1040 return. Unless you need to file a separate 1120 S or C corp or 1065 Business return. And if you do file a separate Business return you still need to amend your personal return to enter the K-1 you will get from the Business return.
