That's a generic C question not a CVI one.
Here are some examples:
char myAsciiChar;
int intValueOfASCII;
char StringRepresentationOfintValueOfASCII[3];
char StringRepresentationOfhexaValueOfASCII[3];
myAsciiChar='A';
//ASCII => Int
intValueOfASCII=(unsigned char) myAsciiChar;
//ASCII => int in a string
sprintf(StringRepresentationOfintValueOfASCII,"%d",(unsigned char) myAsciiChar);
//ASCII => Hexa in a string
sprintf(StringRepresentationOfhexaValueOfASCII,"%X",(unsigned char) myAsciiChar);