All applicants to the National Endowment for the Arts are required to use Grants.gov. (The one exception is the Literature Fellowships: Creative Writing category where this requirement takes effect next year).
The first step in applying through Grants.gov is registration. This is a multi-step process for which you should allow at least two weeks. Registration must be completed before you can apply. See "Register with Grants.gov" for details. Start on this now; don't wait until right before your deadline!