If you post the code you've written we can review it and help you understand what's wrong. If you can't do that your best option is to get a book and study, possibly asking the teacher if he can help you.
//Write a program to find volume of a box
int main ()
int volume, l, b, h;
cout << "Enter Length, Breadth and Height: ";
cin >> l >> b >> h;
volume = l * b * h;
cout <<"Volume is: "<< volume << endl;
If these sorts of questions give difficulty, you need to study the topic, either by working through the tutorial from the start, or by working from whichever textbooks or teaching material is recommended for your course.