Your investors might be people you know such as friends, relatives, and colleagues. Or it could take the form of venture capital, which comes in the form of individuals (often called angel investors) or venture capital firms who invest in businesses that they think are promising. Angel investors will often provide capital and mentoring in exchange for equity in the business, and may require certain conditions such as influencing how to run the company.
If you receive money from friends or family, it's always a good idea to have the terms in writing to avoid future challenges or legal issues. If the money is a gift and not a loan, keep in mind that you may need to report and pay a gift tax to the IRS.