Here is a link to the Michigan Small Business Development Center's Guide to Starting a Small Business. See Part 3, starting on page 20, on ways to legally structure your business.
https://www.michigan.gov/documents/business/Guide_to_Starting_a_Small_Business_271487_7.pdf
The problem with a sole proprietorship (as suggested by FlyingRon) is that YOU are the business and are personally responsible for all liabilities. This is the problem with a partnership, as well, although with a partnership, you and your partner share liability equally.
It generally will be for liability purposes that people form LLCs and corporations. If structured and operated properly, your business assets are kept totally separate from your personal assets. Your personal assets are insulated from the liabilities of your business.
What business structure to form, however, really depends on your personal, business and financial goals. You would be smart to find professional assistance to discuss your businesses and what can work best for you for liability and tax purposes.
Good luck.