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Hi.
The build on podraig's response, here is exactly what you need to do in order to implement a reminder system in iCal.
Fire up iCal from you dock, your applications folder in Finder or by hitting Cmd + Space and entering "iCal". When iCal opens, on the left-hand side (by default) you will see a list of calendars. We need to add a calendar to this list by clicking the "+" icon in the bottom-left of iCal:
Once clicked, you'll get the option of entering the name of a new calendar:
Set the name of your new calendar to whatever you want, such as "Reminders". Next, we need to create a few entries for this calendar. This is simple to do. Firstly, select the date you want to set a reminder for and simply right-click and click "New Event".
Once your new event has been created, it should automatically be set for your new calendar. However, we want to configure a few more options and this is just as easy to do. Firstly, double-click on your new event/reminder to get this window:
From this window you can edit all of the event's settings, including the time of the event and the alarm settings (the import part for you). As you can see it's been assigned to the "Untitled" calendar because I was lazy and didn't enter a name for it. You should see the name of your calendar here; this is import for you also, as it defines this event as being a part of your reminders calendar.
I think you should be OK from here onwards but feel free to ask if you have any other questions.
Cheers.