Hi I'm making a simple calculator program and I've encountered errors that I've never heard of before. No matter what I do, I don't know how to fix this problem

error C2106: '=' : left operand must be l-value

What does that mean? I don't know what I did wrong.
I am using Visual Studio 2008
I haven't done programming in a while so I'm kinda rusty... Please help!
thank you

#include <string>
#include <iostream>
#include <iomanip>
using namespace std;

void main ( )
{
int choice;
double a, b, c;
char an;
cout << "Please enter the operation you want" << endl;
cout << "Press 1 for addition" << endl << "Press 2 for subtraction" <<
endl << "Press 3 for division" << endl << "Press 4 for multiplication"
<< endl;
cin >> choice;
cout << "Please enter the numbers you want to do calculation with" << endl;
while (an = 'y')
{
if (choice == 1)
{
a + b = c;
}
if (choice == 2)
{
a - b = c;
}
if (choice == 3)
{
a / b = c;
}
if (choice == 4)
{
a * b = c;
}
cout << "Here is your result: " << c << endl;
cout << "want to do it again?" << endl;
cin >> an;
}
}