Hello,
I just bought a TFT LCD and an arduino DUE and I was searching for some help on the internet to get it working. I saw some pictures and they where using an extra shield for the LCD screen. Can someone explain if I should also buy such a shield and why they're using it instead of just plugging it in the arduino?
PS: Sorry for the bad english 
what is the model number of your LCD screen?
I don't know the number but this is where i bought it: ebay
You do not need a seperate shield for the LCD you've purchased, it is complete as-is. It was designed specifically for the Mega where as most LCD's use a standardized 34 pin connector. It is these generic LCD's that require a shield to use them with an Arduino.
LCD shields typically provide two things. 1) pin translation from the LCD's single 34 pin connector to the Mega's various 7 connectors. 2) translation from from the Mega's 5 volt level I/O to the LCD's 3.3 volt I/O.
Your LCD should work with the UTFT library, use the display type ili9341 since the hx8357b is not directly supported in UTFT.