The basic syntax for a formula that will work across worksheets is:
=sheetname!cell
For example,
=Sheet1!A1
entered in any cell in any sheet in the spreadsheet will access the data in cell A1 of Sheet1.
The result can be mixed into a formula any way you like. For example, to take the contents of cell D4 from a sheet called Sheet5 and multiply it by 2, use the following formula:
=Sheet5!D4 * 2
To refer to a range from another sheet, just prefix the first part of the range with the sheet name:
=SUM(Sheet1!A1..A10)
will add up the contents of cell A1 through to A10 in Sheet1.