Ah, that's so easy you're going to kick yourself:Any time you're not sure of the syntax of the addressing to another location (another sheet, in this case, or even another workbook), you can get Excel to tell you. Here's one way:
1) Select Sheet1 C3, which already has the SUMIF function laid out.
2) Hit <F2> to start editing the formula.
3) Type in the '/' for division. Do not hit <Enter> yet.
4) Navigate to Sheet2 and select H1. Note that Excel has now added the address for that cell to the formula bar.
5) Hit <Enter>.

Now you can copy the new formula to the rest of the cells on Sheet1!C:C. Oh, wait, your worksheet already does that; why? And I see I should have specified ".../Sheet2!H$1". Anyway, you can use that method to see how Excel handles lookups not only on other worksheets but on external workbooks as well.