When you have an aggregate function, you need a `GROUP BY` statement.  In your case, it would be

    SELECT 
    	LEERLINGEN.LLNR, 
    	UITLENINGEN.LLNR, 
    	LEERLINGEN.VOORNAAM, 
    	LEERLINGEN.TUSSENVOEGSEL, 
    	LEERLINGEN.ACHTERNAAM, 
    	SUM(UITLENINGEN.BOETE)
    FROM 
    	LEERLINGEN 
    	INNER JOIN UITLENINGEN ON LEERLINGEN.LLNR = UITLENINGEN.LLNR
    WHERE 
    	(((UITLENINGEN.BOETE)>25))
    GROUP BY
    	LEERLINGEN.LLNR, 
    	UITLENINGEN.LLNR, 
    	LEERLINGEN.VOORNAAM, 
    	LEERLINGEN.TUSSENVOEGSEL, 
    	LEERLINGEN.ACHTERNAAM;

The GROUP BY clause comes after a WHERE clause but before an ORDER BY (or HAVING) clause on a SELECT statement.  In this case, you want to get the sum of "Boete" for each combination of your other columns, hence the need to group by all of those other rows.

In short, if you have an aggregate function (like SUM, AVG, MIN, or MAX), you'll need to tell the database engine what to do with those non-aggregated columns.  That's a simplistic way of explaining what GROUP BY does for you there.

Incidentally, if you want to find out cases where the sum of all Uitleningen.Boete is greater than 25 (rather than only including the **records** in which Uitleningen.Boete is greater than to 25), you actually have to use the HAVING clause.  Then it would look like this:

    SELECT 
    	LEERLINGEN.LLNR, 
    	UITLENINGEN.LLNR, 
    	LEERLINGEN.VOORNAAM, 
    	LEERLINGEN.TUSSENVOEGSEL, 
    	LEERLINGEN.ACHTERNAAM, 
    	SUM(UITLENINGEN.BOETE)
    FROM 
    	LEERLINGEN 
    	INNER JOIN UITLENINGEN ON LEERLINGEN.LLNR = UITLENINGEN.LLNR
    GROUP BY
    	LEERLINGEN.LLNR, 
    	UITLENINGEN.LLNR, 
    	LEERLINGEN.VOORNAAM, 
    	LEERLINGEN.TUSSENVOEGSEL, 
    	LEERLINGEN.ACHTERNAAM
    HAVING
    	(((UITLENINGEN.BOETE)>25));

Think of HAVING as a WHERE for your groups.  You filter **rows** using WHERE, and you filter **groups** using HAVING.