I strongly recommend trying to use fewer sheets but use tools lke filtering and pivot tables to display the data different ways.

But to create a formula:

1. Click in the cell that you wish displayed a value from the original sheet.
2. Type an equal sign
3. Navigate to and click on the cell that contains the value
4. Press enter

Alternatively:

1. Select the cell that contains the value
2. Copy
3. Navigate to and click on the cell that should display this value
4. Use the drop-down on the Paste button to choose Paste Link

Pay close attention to the Formula bar. (the long white box just below the Ribbon). It will show you the true contents of the active cell. (You will be able to view formulas, not just results.)