Assuming that you will be creating an Add-in for your application.
Import the following:
Imports xl = Microsoft.Office.Interop.Excel
Dim xlApp as xl.Application;
xlApp = Globals.ThisAddin.Application
Dim workbook As xl.WorkBook = xlApp.ActiveWorkbook
Dim worksheet As xl.WorkSheet = workbook.ActiveSheet
Now you can use the properties of your worksheet in the way you want.
However if you want to open a simple workbook, then you can simply do this :
Process.Start("excel")