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Do this by dragging files or folders to the Trash bin icon on the dock. Files you've deleted appear in the Trash bin. Click the Trash icon on the dock to see what's in the Trash. This is where you'll be able to access the option to permanently delete files you've moved to the trash. ” A dialog will appear, asking “Are you sure you want to erase the items in the Trash permanently using Secure Empty Trash?” Click “OK” to remove. This process can take anywhere from a few minutes to several hours, depending on how big your Trash bin is. If you'd prefer to delete all of the files on the computer instead of just a select few, you can format the hard drive. This option will completely destroy all data on your computer and then reinstall Mac OS X. It can take several hours if you have a big drive.

Summary:
Move files and/or folders to the Trash. Open the Trash to view deleted files. Click the Finder icon on the dock, then open the Finder menu. Select “Secure Empty Trash. Format your hard drive.