How to use cronjob in codeigniter?

That is probably a question for your system administrator rather then a CodeIgniter question since it depends on your OS (Linux, Windows, Solaris etc.) and also your distribution/version. There are simply a lot of different ways depending on your tools.

Also, you fail to mention what your controller/method is supposed to do. Is it enough to "download" it, or do you want to send parameters or process the result in any way?

On my Linux Ubuntu 14.10 I could for example execute
# crontab -e

This will edit the list of cron commands tied to my user (editor depends on what is installed in your system). I would and then add the following line
0 * * * * wget http://www.mywebsite.com/welcome/auto

This will "visit" your website once every hour and download the output, however the output would simply be discarded.

Kind regards,

Daniel Sahlberg