I've found here datasheet:
http://www.geeetech.com/Documents/LCD1602%20White%20Datasheet.pdf
But i can't find what type of resistor i need.
That is not the datasheet for your LCD module but it looks fairly typical.
Look in the middle of page 6 in section 7. The nominal current for the LED is 110mA and with that current the voltage drop across the LED will be 4.2 Volts. Since you are using a 5 Volt supply the series resistor will have to drop 0.8 Volts. The required resistance is 0.8 V divided by 0.110 A which gives 7.27 Ohms. I would try a 10 Ohm resistor but you could probably use a much larger value and still have a usable amount of backlight.
Don