fopen() opens a file, the second argument should be a string, either "w" or "r" for write/read
reading/writing individual characters you should look at fputc()/fgetc() or better fwrite()/fread().
simplest but not the most efficient way:
1. open the file for read only - explained above, get a file stream to read.
2 set an integer variable as a counter = 0.
3. while (fgetc(<file stream variable>)!=EOF) counter++;
4. close the file
5 display the number of characters (use the counter varaible) with printf
The standard library functions you need are: fopen, fclose, printf, fgetc
So you can read your documentation on how to call each of those