Might help if you know how to login to your server once you have your hands on a terminal program.
Type,
ssh
YOURUSERNAME@YOURDOMAIN.com
Then hit return at which time it will ask for your password. It will not show your password as you type it.
As cd34 said, type,
crontab -e
On most of the systems Vi is the default editor. I don't use Vi enough to remember how to use it sometimes so I pull up this page.
http://staff.washington.edu/rells/R110/#basics5
Sometimes Pico is the editor. This one is for humans to actually use and works like a regular word processor with the commands listed at the bottom of the screen.
Typing man crontab will pull up the manual for crontab. If you are really new to all this you may want to start off by typing man man to read the manual on using the manual.