Imadou93:
I know this, and I have tried it, but this only works when you type something in the serial monitor which is not my question!
My question was, what should I write in my TERMINAL (after connecting it to my board using telnet), to send instructions to my board?
If 'TERMINAL' means your telnet client, then what you type in is the character or characters that you want to send the Arduino. That's so obvious that I can't believe you need to be told, and perhaps it's not what you're trying to ask. In that case you need to make your question clearer.