How to Allocate dynamic memory to string data type...in c++

Answer 1

Not sure about String but you can always create a character array.
char* value = NULL;
value = new char[20];

Answer 2

How can I know that user gone enter 20 letters only...He can enter more than 20 alos

Answer 3

You can keep larger values as well, but if the user enters something bigger than that then you have to declare and initialize a whole new memory which can accommodate the content.