The entrepreneurial sector is the smallest of the four in terms of current employment numbers. But it will most likely become an increasingly important source of jobs in the future, given changes in the other three sectors, where employment opportunities may be flattening or decreasing.
If your company has unique and valuable intellectual property, you will need to protect it legally. This can be done using a combination of copyright, trade secrets, and patents. To determine if you need to patent your invention, see the US PTO flowchart.
When first starting out, most entrepreneurs raise money from the three Fs (friends, family, fools). Once you have exhausted their resources, you would move on to angel investors (high-net worth individuals, may invest as individuals, or as part of a group), then move on to venture capitalists (who have certain criteria for investing that are set by the limited partners, usually with certain timelines and specific industries).