Assuming that this is for a school assignment, I'll try to provide the proper kind of assistance.
First of all, think of how large you should make your array when the program starts. Once it's made, it can't be changed unless you make a new one and copy contents over.
To read the characters from the file, have a look at the fscanf manpage.
There is a function that you will find very useful for this assignment in ctype.h.
As far as how to easily navigate your array or file, have a look in your text book's index under "pointer arithmetic".
Hope this helps,
~arker
