How do use string in c++?

Answer 1

string str = "cookies";

cout << str;

Answer 2

The + operator is overloaded so for concat you simply do :
string str1;
string str2;
string str3 = str1+str2;

Answer 3

declare a variable first:
string x;
assign a value for this variable:
x = "value is here";
if you want to show this value on screen
cout << x;

Answer 4

concatination , copy substring