Im am new to Access also and dont really know any V.B. but what the guy above is telling you to enter the VB code. Try these things on a copy of your database in case it naffs up!
Go into design view, select the combo box you have made as explained by the other guy. Right Click, Choose Properties, Select 'Build Event' Choose 'Code Builder' your now in VB - Simply enter the Code which the guy has given you between 'BEFORE SUB' and 'END SUB', choose Save and your laughing!