Hi, I have already filed my 2018 personal taxes but I now need to file my small business taxes. Should I create an entirely different account if I have a TIN?

[LisaS]

It will depend on what "entity" your small business is.  TurbTax Business product is a separate product used to files Partnerships and Corporations.  If you did not form a partnership or corporation then you will need to amend your personal return and add the a Schedule C form to the return.  This is where business income is reported when you have not formed a partnership or Corporation.