just set the record source under the form properties. If you set the record source to a query you can enter a criteria in the query so that only records matching that criteria are retrieved. Your criteris will be set to the Combobox so that only records matching the value on the combo box are retrieved.
You can also set it through code"
Dim SQL as string
SQL = "Select * from Table Where Somefield = " & combobox.value
Form.Datasource = SQL
Form.Requery