Hi,
the answer depends on which sheet your formula is in:
If the formula is in Sheet2:
=VLOOKUP(Sheet1!B7,C4

33,2,)
or
=LOOKUP(Sheet1!B7,C4

33)
If the formula is in Sheet1:
=VLOOKUP(B7,Sheet2!C4

33,2,)
or
=LOOKUP(B7,Sheet2!C4

33)
If the formula is in Sheet3:
=VLOOKUP(Sheet1!B7,Sheet2!C4

33,2,)
or
=LOOKUP(Sheet1!B7,Sheet2!C4

33)
So as Luke indicated the best way to build formulas is to point and click.
In otherwords start by typing =VLOOKUP( and then click the sheet tab and
then select the cell(s) you want and continue.