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

Transmit String

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