How do I create a new post upon registration with the users first and last name as title

The get_userdata function is what you are looking for. So for the display_name it would be

$user_info = get_userdata($user_id);
$profile_display_name = $user_info->display_name;

And for first and last name it would be

$full_name = $user_info->first_name .  " " . $user_info->last_name;