If Function and cin

Im very new to c++ and trying to write a simple game to learn some of the basic mechanics and i'm having trouble with my code,

How do I define the function WantToPlay so that when a player enters "yes" it replies fantastic, and "no" replies boooo?

My current code gives the error that WantToPlay must be of bool type, but i'm not sure how to use a boolean function to rectify it?

Thanks!

using std::cout;
using std::cin;
using std::endl;
using std::string;


string leader;
string WantToPlay;

int main()
 {
    std::cout << "Dungeon Master: Welcome to my dungeon, traveller! What is         your name?" << std::endl;
cin >> leader;
cout << "Dungeon Master: Well hello, " << leader << " I have a little quest for you if you're interested?" << endl;
cout << "(Please enter ""yes"" or ""no"")" << endl;


cin >> WantToPlay;

if (WantToPlay = "yes")
    cout << "Fantastic!" << endl;
else if (WantToPlay = "no")
    cout << "boooo" << endl;




return 0;


}