How do I solve this?

[asy]

Write a complete C++ program to do the following:
• Get three input from customer that identifies the type of promotion (1 or 2), type of package ( 1 or 2) and with or without mini package ( A or B).
• Display the appropriate message when the customer enters an invalid input and the customer can re-enter the choice,
• Compute and display the monthly total cost before and after the 6% government tax is added in a receipt (text in the program)..

[Ganado]

Input from the user can be obtained by using std::cin
See example in http://en.cppreference.com/w/cpp/io/cin

Displaying appropriate messages for invalid input is a bit more complicated, but you'll need to use if-statements to check the input, and then a loop if the input isn't what it should be (ex, if user entered a number greater than 2, or less than 0).
See examples in http://www.cplusplus.com/doc/tutorial/control/

Not enough information given for the third point.

If you want people to help you, you should post what you've tried yourself, and any specific errors or problems you have.