Finding the IC of a plaintext can be incredibly tedious, especially if the the text is long. There are some good online sites that will calculate the IC for you, here is a good one that not only will calculate the IC, but will also give you other statistics and allow you to manipulate the text in some ways: http://rumkin.com/tools/cipher/manipulate.php.

You can also write your own program to calculate the IC.
Here is a java program I wrote that will do the job.

/*
This program measures the index of conincidence on a given text.
Date: First draft 11/2/09
*/

import java.io.*;
import java.util.*;

public class indexOfCoincidence {

    public static void main(String [] args) throws IOException {
            long start = System.currentTimeMillis(); // start timing
            BufferedReader f = new BufferedReader(new FileReader("indexOfCoincidence.in"));
            String plaintext = f.readLine();
            int[] charfrequency = new int[26];
            int i = 0;
            int j =  0;
            int textlength = 0;
            while(i < plaintext.length()) {
                char c  = plaintext.charAt(i);
                j = (int) c - (int) 'A';
                if(j >= 0 && j <= 25) {
                    charfrequency[j]++;
                    textlength++;
                }
                i++;
                j = 0;
            }
            double ic = 0;
            for(int k = 0; k < 26; k++) 
                ic += charfrequency[k] * (charfrequency[k] - 1);
            ic /= textlength * (textlength - 1);
            System.out.println("The index of coincidence is " + ic + ".");
    }
}