> what tools could I use to start developing this program
A text editor and a compiler is a good start.
> what's the best way to format
An array that stores a count of the characters that occured, ie
Code:
int freq[UCHAR_MAX - ' '] = {0}; /* more than enough for all the printable ascii characters, assumes ascii of course */
/* go through the string you read and add the printable characters to the array (with isprint() from ctype.h), then carry on...
just make sure you don't go out of bounds of the array */