Hi! I really new to Arduino and I don't know how to go about this school project

: I need to display a countdown clock on my Parallax Serial LCD screen. I've gotten as far as being able to write to the LCD screen, but I don't know how to structure the timer. Here is what I've got so far: Thanks for your help!
const int TxPin = 6;
#include <SoftwareSerial.h>
SoftwareSerial mySerial = SoftwareSerial(255, TxPin);
void setup() {
pinMode(TxPin, OUTPUT);
digitalWrite(TxPin, HIGH);
mySerial.begin(9600);
delay(100);
mySerial.write(12); // Clear
mySerial.write(17); // Turn backlight on
delay(5); // Required delay
mySerial.print("BEGIN TEST"); // First line
mySerial.write(12); // Clear
delay(1000); // Wait 1 second
}
void loop() {
}