In order to accept donations through Causes, a nonprofit must be a U.S. based 501(c)(3) that is also registered with GuideStar, the database where we get information about nonprofits.
Nonprofits: Guidestar has a fairly simple registration process detailed in these FAQs. Online donations & sharing information with GuideStar partner organizations (like Causes) must also be enabled in your GuideStar profile; to review or update your organization's settings you can login on their site here.
Pro-tip: Do more with your campaign than create a fundraiser. Consider starting a petition or pledge. These actions will build your supporter base on Causes and later lead to fundraising.