Question about the User model?

Question

Laravel

Hello
After installing laravel, there is already a User model

<?php

namespace App;

use Illuminate\Foundation\Auth\User as Authenticatable;

class User extends Authenticatable
{
    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'username',
        'email',
        'password',
        'first_name',
        'last_name',
        'avatar',
        'user_group'
    ];

    /**
     * The attributes excluded from the model's JSON form.
     *
     * @var array
     */
    protected $hidden = [
        'password', 'remember_token',
    ];

It extends Authenticatable which connects interfaces for authorization etc.

<?php

namespace Illuminate\Foundation\Auth;

use Illuminate\Auth\Authenticatable;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Auth\Passwords\CanResetPassword;
use Illuminate\Foundation\Auth\Access\Authorizable;
use Illuminate\Contracts\Auth\Authenticatable as AuthenticatableContract;
use Illuminate\Contracts\Auth\Access\Authorizable as AuthorizableContract;
use Illuminate\Contracts\Auth\CanResetPassword as CanResetPasswordContract;

class User extends Model implements
    AuthenticatableContract,
    AuthorizableContract,
    CanResetPasswordContract
{
    use Authenticatable, Authorizable, CanResetPassword;

}

The question is actually how to access the data in the model?
write a function that will get the username

public function getUserName()
    {
        return $this->first_name;
    }

And this function can't get user data.
Tell me what am I doing wrong? How to access data from users table in User model?