The first one is a PHP problem. Something is possibly misconfigured or missing - best guess based on experience.
I recommend following the advice given by the third one, namely reading the user guide. It should help you solve the second one.
Your 3rd link is showing the default page of CI, which means you installed everything correctly for that. Without more information on your config.php, main index.php and other Codeigniter setup files, it’s hard to guess what’s causing your problem.