First you have to check if the cron daemon is running and if you as a user are allowed to run cron jobs, but don't worry too much, since Linux enables that by default (just check if something doesn't work). To write your own crontab you will issue the command
it will open a vi session in which you can write your own cron jobs. Then save and quit and the system will store the proper file in the proper location automatically. Check man crontab for other options of the crontab command (for example to display or to remove the current crontab).
For the format of a crontab line, that is how to specify a cron job see
it describes the crontab format and gives some useful examples. If you're in trouble feel free to ask.