Hi,
It depends on what you want to do exactly.
One way is to use office components in you App.
(this)
Another one is to create csv files. (Although CSV files are not Excel files but Excel can open and edit them). So it is good when you just want to write some data and view them formatted in excel.
Another way could be using SQL Server.
this
And the last way is using Ado.Net
this
Hope those can help you.