If your program truly has nothing useful to do during user input then you can use code like...
Serial.print("Enter a number >");
while(!Serial.available()); //wait forever for a user input
int i = Serial.parseInt();
Serial.println(i);
But even the slow Arduinos are running 16 million instructions per second waiting for your slow fingers to reach the keyboard. There is always something useful to be done, such as checking the motors didn't drive past their limits or blinking the blinky lights to let you know it is waiting for you. So anything but the simplest school project should use Robin's method which does not tie up the processor waiting.