Go to builder.movn.me and log in with your Movn login details or create a new account if you're new. If your account isn't yet linked to an organization, you'll be prompted to create one right then, that's what lets you start building apps.
The other way in is by invitation: if someone adds you as an admin of their organization, you'll get an email invite and land inside their organization directly, no need to create your own.
You're not limited to one organization either: you can create another anytime from Profile > Organizations > Create Organization, useful if you're running apps for more than one club, event, or brand. Every organization you belong to shows up on your dashboard, where you can switch between them. See 1.5 to manage an organization you already belong to.