Open Excel File on a specific worksheet

Like this:

 using Excel; 

 Excel.Application excelApp = new Excel.ApplicationClass();

  // if you want to make excel visible to user, set this property to true, false by default
  excelApp.Visible = true;

 // open an existing workbook
 string workbookPath = "c:/SomeWorkBook.xls";
    Excel.Workbook excelWorkbook = excelApp.Workbooks.Open(workbookPath,
        0, false, 5, "", "", false, Excel.XlPlatform.xlWindows, "",
        true, false, 0, true, false, false);



// get all sheets in workbook
   Excel.Sheets excelSheets = excelWorkbook.Worksheets;

  // get some sheet
 string currentSheet = "Sheet1";
    Excel.Worksheet excelWorksheet = 
        (Excel.Worksheet)excelSheets.get_Item(currentSheet);

 // access cell within sheet
  Excel.Range excelCell = 
        (Excel.Range)excelWorksheet.get_Range("A1", "A1");

Hope this helps

MDSN info here