To start a business, all you need to do is hang up a shingle. See Characteristics of Business Entities for a chart comparing different types of business forms.
Partnerships are also easy to create.
Again, there are few legal formalities. General partnerships usually do not need to register anywhere except maybe at the county level. Limited partnerships are more like corporations and usually require registration at the state level and the services of a registered agent. When a corporation enters into a transaction, it is the corporation and not the shareholders who is responsible. When starting up, a bank may require a personal guaranty since your company doesn't yet have a credit history but, as a rule, a shareholder's liability is limited to the amount invested into the company.