To begin the export process of your iCal calendar, open up iCal and navigate to File>Export>Export...

Enter the name of your calendar, select where you want to store it, and press Export. 

Next, go ahead and open up the Google Calendar where you intend to import the information to.
Locate the settings symbol on the top of your screen and select settings.

In the settings page, scroll down to locate Import & Export. Access the import half of that section and press Select file from your computer. You will then be able to locate your file from your computer.

Select your file and click Open.

Once selected, choose from the drop down menu what calendar section you would like the file to go into. Select import. 

The information from your iCall calendar will then be automatically entered into your Google Calendar.