Rohan already answered the question. If you want it to be the first Table then add it first. However, I would suggest referencing by name instead of relying on indexing being in a specific order.
You can do this:
myDataSet.Tables["SomeTable"]
instead of
myDataSet.Tables[0];
Then you know you're accessing the right datatable when there are more than one in a dataset and ordering is not important.