I want to set up cron jobs.. how can i do that

Hi,

I realized that my contact invitations was not been sended, and taking a look in the admin component i found a tab named "email activity" where i see that i need to set a cronjob, but i don´t know how to do this..

Can you giveme some inputs about this? or theres a tutorial to do this?... and for another things or actions for the users where i need to create this cronjobs.

Whats the cronjob command to send this email contact invitations that the users made to invite some others?

Thanks!