Hi Rajkumar,
Thank you for contacting Aspose support.
You must be referring to the second worksheet while retrieving the instance of worksheet from the collection. Please note, the WorksheetCollection has 0 based index so when you call the following statement, it will get the second worksheet from the collection.
C#
var sheet = book.Worksheets[1];
Replace the 1 with 0, and it should retrieve the first worksheet for further processing.
C#
//Initialize Workbook
var book = new Workbook();
//Get instance of first worksheet
var sheet = book.Worksheets[0];
//Get instance of first worksheet's cells
var cells = sheet.Cells;
//Import DataTable to first worksheet cells
//Choose the best overload that suits the requirement
cells.ImportDataTable(dataTable, true, "A1");
Regarding the other inquiry, a DataSet is the collection of DataTables so you can easily iterate over the DataTable collection in a DataSet to import the data to worksheet cells as elaborated above.
Please feel free to write back in case you need our further assistance with Aspose APIs.