You use your SS# if you are a non-incorporated sole proprietership and pay your taxes that way. You need a new tax ID if you create a seperate legal entity to do business under. Basically, this means a C corporation. Other entities probably don't require a different tax ID, like an S-corp, limited liability company and limited liability partnership, depending on how you plan to pay your taxes.. . On that note. If you are engaging in a business and plan to pay taxes as a business, you need an accountant. The more formal your business or the more complicated your tax entity is, the more you need a good accountant. The more money you make, the more you should consider the liability protection and/or tax advantages of different legal entities.