Do a search on the Board for Cascading combo -- there are plenty of examples. Also, think about the structure. If the second field is one to one with the first (eg, pick a Category and NEVER more than one member), you can keep the second field in the same table. If you will have multiple items, you need a related table that will let you add one record per item.
Access 2007 does have multi-value fields (to make it compatible with some of the list features in Sharepoint) but this feature is not compatible with earlier versions of Access.