Windows: hold Alt and type 0241 on the numeric keypad for ñ, or 0209 for Ñ
Mac: press Option+N, then type n for ñ or Shift+N for Ñ
Linux: use the compose key or type Ctrl+Shift+U, then 00f1 for ñ or 00d1 for Ñ
iPhone/iPad: press and hold n or N, then select ñ or Ñ
Android: press and hold n or N, then select ñ or Ñ
Spanish keyboard: press the key for ñ directly
HTML: use ñ for ñ or Ñ for Ñ
Unicode: ñ is U+00F1 and Ñ is U+00D1