C#: How to access an Excel cell?

I am trying to open an Excel file and populate its cells with data? I have done the following coding so far.

Currently I am at this stage with the following code but still I am getting errors:

Microsoft.Office.Interop.Excel.ApplicationClass appExcel =
                new Microsoft.Office.Interop.Excel.ApplicationClass();
try
{
    // is there already such a file ?
    if (System.IO.File.Exists("C:\\csharp\\errorreport1.xls"))
    {
        // then go and load this into excel
        Microsoft.Office.Interop.Excel.Workbooks.Open(
            "C:\\csharp\\errorreport1.xls", true, false, 
            Missing.Value, Missing.Value, Missing.Value, Missing.Value,
            Missing.Value, Missing.Value, Missing.Value, Missing.Value, 
            Missing.Value, Missing.Value, Missing.Value, Missing.Value);
    }
    else
    {
        // if not go and create a workbook:
        newWorkbook = appExcel.Workbooks.Add(XlWBATemplate.xlWBATWorksheet);
        Microsoft.Office.Interop.Excel._Worksheet excelWorksheet = 
            (Microsoft.Office.Interop.Excel._Worksheet)
                newWorkBook.Worksheets.get_Item(1);
    } 
i++;
j = 1;

j++;
objsheet.Cells(i, j).Value = "Tabelle: " + rs.Fields["Table_Name"];
j++;
objsheet.Cells(i, j).Value = "kombinationsschluessel:FALL " 
                                + rs3.Fields[1].Value;
j++;
objsheet.Cells(i, j).Value = "Null Value: ";
j++;
objsheet.Cells(i, j).Value = "Updated with 888";

These are the top 2 errors I am getting:

Error 1 An object reference is required for the nonstatic field, method, or
        property 'Microsoft.Office.Interop.Excel.Workbooks.Open(string, object,
        object, object, object, object, object, object, object, object, object,
        object, object, object, object)'

Error 2 The name 'newWorkbook' does not exist in the current context