Take a closer look at this section of your code:
char numString[TAILLE_NUM_UTI];
//place le pointeur au debut du fichier
myFile.seek(0);
//retrieve the ID number of the line
for(i=0; i<TAILLE_NUM_UTI-1; i++) numString=myFile.read();
numString='\0';
Specifically:
char numString[TAILLE_NUM_UTI];
for(i=0; i<TAILLE_NUM_UTI-1; i++) numString=myFile.read();
numString='\0';
Last I checked, read() doesn't return a pointer, it returns a single byte. That shouldn't even compile...