DropDown menus in Excel

Help. I am putting drop down menus in my Excel worksheet which is working fine but on some of the options I need to "filter" what is given in other menus. For eg, First drop down gives option and depending on what option you choose, another drop down menu gives options. Say I choose "Not Applicable" in the first menu then I want the second menu to automatically put in "Not Applicable" to prevent something being chosen. Or, I choose and option in the first drop down and then the second drop down will automatically give me the choices available for that option (ie. colours).
Can you help?