To create a new query, click the
button. If the current query has changed, you will be prompted to save or discard those changes.
Otherwise (or after saving/discarding changes) a sheet will appear asking you to select the
base table for the new query.
The base table is the table you are primarily interested in working with. You will also be able to select fields from any tables that the base table has a relationship to. Once a table is selected, you may now begin editing the query.