Suggestios for Dropdown list Selection fro Showing Message

Question

Dear Friends ..,

This is Narasimha Murthy ... I am Associate Software Engineer From Blue Frog Mobile Technologies ., Visakhapatnam 

I Need Suggestion from you ..,

How to show message pop up for when i select the drop down list ?

Scenarios : 

state : ddlstate ---> Andhrapradesh , Assam , ............ , uttaranchal , uttarapradesh ... like that ,

District: ddldistrict ---> Srikakulam ,  Vizianagaram , .............. When i select ddlstate as Andhrapradesh

Mandal : ddlMandal ----> Rajam , Palasa .., like that  when i select ddldistrict as Srikakulam ..,

In this way ..,

if i want to validate Message pop up to show when i select ddlMandal without selecting ddldistrict ..,

Please send the Details 

Thanks ,

T V S Narasimha Murthy