Durban
I'm not too sure I understand, is the cell that you want the result in random???
Let me try and explain my understanding. If you want to produce say an invoice on sheet2 from a list of parts on sheet1 for example then build your table of parts and prices on sheet1 (Part# in column A, Price in column B and we'll say that list extends down to row 20, A1:B20). Do this first, part# in A1:A20 and the corresponding prices in B1:B20
Now go to sheet 2 and we will imagine the area set aside to enter part# is in column A (starting at row 6) and you want the price to appear in column B (again starting at row 6). So in cell B6 enter:
=VLOOKUP(A6,Sheet1!$A$1:$B$20,2,FALSE)
Now, without a part# in A6 this will return #N/A as it cannot find the blank cell in the list on sheet1 but if you now enter a part# from the list in A6 it will show the price in B6. You can copy B6 down as far as you want to cells below to make your list of invoice items.
Not wanting to confuse at this early stage, but the #N/A is annoying when you have no part# in column a so you can add an IF and error check to return nothing until you have a part#
=IF(ISNA(VLOOKUP(A6,Sheet1!$A$1:$B$20,2,FALSE)),"",VLOOKUP(A6,Sheet1!$A$1:$B$20,2,FALSE))
But that's for next time 
Regards
Nick Hodge
Microsoft MVP, Excel
Southampton, UK