@AndrewStacey: There's a list in the right of the text area. You can find the tables names (e.g., Users) and the fields (aka columns) of each table, represented by the bullet items (e.g., Reputation). Let's say you want to display the username and the reputation, sorted by reputation:
select DisplayName, Reputation from Users order by Reputation desc
(ouch, the chat trims the leading spaces!)