Hi Tanja,
instead of directly selecting some data from the DB tables you should read this information via the BOL (c.f BOL Programing - CRM - SCN Wiki for a short introduction).
Using the transaction GENIL_MODEL_BROWSER you can browse through the data model and find the suitable relations for reading the required data. In your case one possible approach to access the data is BTAdminH->BTHeaderBillingSet->BTBillingSetBuAg. Once you have the BuAg BOL object you can simply read the TOFPAYM property.
Best,
Christian