While you can enter the correct formula, specifying the "SheetN!cell" format, it might be something that might not be too memorable.
So, Excel has a simple way of doing that with the mouse.
Just click the box you want updated, then press the "=" key, and then click the cell you want the data to be retrieved from.
If that data comes from a different sheet just go to that sheet to then click on that sheet and then the cell.
Note: I'm pretty sure that even works across workbooks, too. That if you need to get data from another workbook, just press the "=" key and then open the other workbook to click on that cell.
Any of the required formula formatting and syntax gets put in place for you. Try it out.