Do you think that the potenciometer is the problem?
No, because you can see the black rectangles on the first line of your display.
Your display is not being initialized properly. Are you sure that the program code is being downloaded into your Arduino?
Also, have you read the very first thread in this section of the forum? It is the one that is 'sticky' so it always appears first, the title is 'bold' so it stands out, and it says "Warning to users of some vendors LCD keypad shields".
The particular shield that initiated that thread is a DF Robot LCD Keypad shield although it might be an earlier model than the one you have.
Don