Can you confirm the database type please? Are you connecting to a relational database or an OLAP database?
When you have established connection to a Data Source, Tableau would read through the data available and present you the dimensions and measures identified. However, it is possible to hide unwanted dimensions or measures by right clicking on the dimension or measure and selecting 'Hide'.
When you have made all the necessary changes (hide/unhide, define alias for fields, create calculated fields etc.), it is possible for you to save this definition as a Tableau Data Source for the benefit of other users or for your own future use when connecting to the same database.
You can also use Custom SQL to decide which columns you want to add to your workbook. The following link provides further information on this:
Hope this helps!