string phrase = "here I am";
string sfile;
// open a file for reading
while(/* not to the end of the file */)
{ // read a line
  // add it to sfile
}
// now, sfile contains the entire file
for(int i=0;i<sfile.size();i++)
{ if(phrase == sfile.substr(i,phrase.size()))
  { //the phrase has been found
  }
}

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
    int length = 0;
    char* p_data;

    cout << "Enter the name of the word file: ";
    string filename;
    getline( cin, filename );
    ifstream infile( filename.c_str() );
    if( infile.is_open() && infile.good() && !infile.eof() && !infile.fail() && !infile.bad() )
    {
        infile.seekg( 0, ios_base::end );
        length = infile.tellg(); // get the file size
        infile.seekg( 0, ios_base::beg );
        p_data = new char[length];
        if( p_data )
        {
            infile.read( p_data, length );
        }
        infile.close();

        string s( p_data );
        delete [] p_data;
        p_data = 0;

        cout << "Enter the phrase you want to find in the text contained in file " << filename << ": ";
        string phrase;
        getline( cin, phrase );

        basic_string <char>::size_type phrase_start = 0;
        phrase_start = s.find( phrase );

        if( phrase_start != std::string::npos )
        {
            cout << "Phrase found starting at character position: " << phrase_start << endl;
        }
        else
        {
            cout << "Phrase not found..." << endl;
        }
    }

    return 0;
}