Having looked at your code I'm still struggling a bit to follow what is happening. You seem to have missed out some important steps, unless I haven't understood it properly.
First: you need to decide what range of input numbers the calculator can handle. For example, 0 to 99? 0 to 999? Do you want to include negative numbers?
Anyway, once you've decided that, you need to understand that the A/D converter with the potentiometer on it will create an integer value in the range 0 to 1023. So you need to convert that range to your input number range. The map() function would be useful for this:
sensorValue = analogRead(A0);
firstNumber = map(sensorValue, 0, 1023, 0, 99); //valid number range is 0 to 99
lcd.print(firstNumber);
So, firstNumber ends up with the first number stored in it, and it gets printed out on the display as you change the potentiometer.
There is a LOT more for you to work on yet, but does that help to get started?
Now, some advice (if I may). You are trying to do too much at once. I can tell from your code that you are a beginner to Arduino and 'C' (there are several mistakes in your code), so the best thing to do is build up this program in tiny steps.
First, just create a program that does only what I have written above (it isn't complete - so don't cut-and-paste it!). In pseudocode, it would look something like this:
int sensorValue = 0;
void setup()
{
// put your setup code here, to run once:
}
void loop()
{
sensorValue = analogRead(A0);
firstNumber = map(sensorValue, 0, 1023, 0, 99); //valid number range is 0 to 99
lcd.setCursor(0,1);
lcd.print(firstNumber);
Serial.println(firstNumber);
delay(100);
}
Once that is working, sort out the obvious bug in it (to do with overwriting the previous number).
Then, add code to check for the button press each time round the loop. Think first about whether you want to check for "Is the button down?" or "Has the button been pressed since the last check?". It's a very important difference! Also, think about whether you want to respond to pressing the button or releasing the button. What if the user presses the button, and then turns the potentiometer before releasing it?
Another point: do you really need two buttons, one to store and one to move on? I don't think so. Store the number each time round the loop. Use a button to move on to the next step. And @TomGeorge's suggestion to use the potentiometer to select the operator is excellent and makes for a very intuitive interface.
Anyway, you get the idea: don't try to make it all work at once. Get one part working first, then add to it bit by bit.
It's more complicated than you think! But it is an excellent starter project.