For a SELECT statement, you have to specify the list of what will be selected. It can be objects of three types: fields of the tables and views marked by the user, aggregate functions, and arbitrary constructions entered manually.
To specify what field the function is applied to, mark the field or a group of fields in the list, then select the function that will be used and press the button opposite the group of functions with an arrow on it.
Use the DISTINCT button in case you have to select only unique values for the marked field. You can also specify aliases for columns of the results. To set or clear them, use the AS button and the edit box next to it.