You're talking about business structure.
A sole proprietor is simply you. You use your SSN to register and you have 100% personal liability if you take out a business loan, have terms from your vendors, get sued by a customer or employee. There is not any real limit to what can be taken from you personally if chit hits the fan. The advantage to being a sole proprietor is you can usually be set up in a matter of hours and for free or very little charge. Complete an IRS form SS4 (Google it)
An LLC or Corporation offers you personal protection from Liability as long as you follow certain rules. You're esentially creating another business entity that you and partners will own, but it's technically it's own person in the eyes of the government. If something happens like a lawsuit, your personal assets will likely be protected.
You can set up a corporation or LLC on sites like MyCorporation.com or LegalZoom. There is a cost and it will take some time. You can also speak with your accountant or lawyer for help.
If I understand your question correctly, you can't just select LLC/Corporation on a form if you have not actually set one up.