You may first need to turn off the UART functioning as a serial console by following the instructions here.
The default setup of the UART is 115200-8-N-1 (115200 baud, 8 data bits, No partity, 1 stop bit and no flow control).
What is current port setup?
stty -F /dev/ttyAMA0
This will send "hello" from the serial port:
echo "hello" > /dev/ttyAMA0
press Enter to send it. (The last character is a zero not 'O').
To send with a carriage return and line feed:
echo -e "hello world\r\n" > /dev/ttyAMA0
Set Baud Rate
stty -F /dev/ttyAMA0 57600