Windows: hold Alt and type 0169 on the numeric keypad
Mac: press Option + G
iPhone/iPad: press and hold the letter C, then select ©
Android: press and hold the letter C, then select ©
HTML: use © or ©
Unicode: U+00A9
Microsoft Word: type (c) and let AutoCorrect convert it to ©
Google Docs: insert it from special characters or type (c) if AutoCorrect is enabled