Arduino Uno serial communication using USB-to-TTL converter
How to send data(Hello World) from Arduino Uno to PC using USB to TTL/UART(PL2303 4 wires)? 


Serial.print("Hello World");


That is, assuming it's connected to pins 0/1. Otherwise you would have to use SoftwareSerial.


  why I need to press the reset button during upload a program using PL2303?


Because it lacks a DTR signal to press the reset button for you (resetting runs the bootloader).