Problem solved.
help was from arduino how to use if statement
added sentence
if(digitalRead(resetbutton) == HIGH) { count = 20; lcd.begin(7, 2); lcd.print("counter"); lcd.print(count); }
SOLVED ! 
hi,
i have done the code, but i need to reset counter , when the button is pressed.something like...
if(digitalRead(resetbutton) == LOW) { count = 0; }...
but as i'm beginner, i hae little stucked there ..
here is my working code (yes, it has 1 disadvantage, it does not stop counting when reaches zero 
)
i have did button to pin 8, so only need somewhere to write IF sentence. thought it will be easy as excel, but it's not ...
it uses infrared gate and counts down from 20. when button pressed should counter reset back .
CODE ::::
//
#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
int count =20;
int gate = 7;
int resetbutton = 8; // need to input somewhere in code , to reset the counter when button pressed, when low/or higt
bool item_detected = false;
void setup() {
// set up the LCD's number of columns and rows:
lcd.begin(7, 2);
// Print a message to the LCD.
lcd.print("counter");
lcd.print(count);
pinMode(gate,INPUT);
}
void loop()
{
int val = digitalRead( gate );
if( (item_detected == false) && ( val == 0 )){
item_detected = true;
count--;
updateCounter();
}
else if( (item_detected == true) && ( val == 1 )){
item_detected = false;
}
}
void updateCounter(){
lcd.clear();
lcd.setCursor(0,0);
lcd.print("Left");
lcd.setCursor(0,1);
lcd.print(count);
}