How do I wipe the hard drive of an old mac before I throw it away?

I am living abroad and there is no Apple recycling program here. We do have municipal recycling for electronics. It is an old shell back i-mac. Pretty sure it has credit card data and stuff like that on it. How shall I remove the data from the hard drive so that it is safe to throw away? This should be an easy question for all the brilliant tech people on this site. Help!