This isn't a question that makes any real sense as asked: the DataSet class isn't one that you would normally write any source code in - it's a class that is provided by .NET to interface between a data source and the DataTable object it returns. Yes, it can be used to provide the data for Crystal Reports, but that's not really a DataSet function - your code provides the SQL (or other data source) code to fill the DataTable / DataSet object(s) with the data you want CR to work with rather than leavign that to teh DT / DS itself.
I'd start by reading this:
Creating Crystal Reports using C# with Datasets[
^] and then branch out via google to find more in-depth tutorials on what exactly you are trying to achieve when you have the basics right.