Raspberry pi & arduino connection

Hi ! I want to connect my raspberry pi (where openhab is installed) with an arduino via a wire in order to light a single led. What binding I should use, serial or GPIO and how can I use GPIO?

If you are using a single wire you need to use the GPIO binding. Documentation for how to use it is on the wiki.

you can also use some usb ftdi adapter board to connect to the arduinos rx/tx pins.