One of the easiest way to seed users/admin-user into the system is by running the following on the server when it starts:

Meteor.startup(function() {
  if (Meteor.users.find().count() === 0) {
    Accounts.createUser({
      username: 'test',
      email: 'me@til.codes',
      password: 'password'
    });
  }
});