how to validate using asp.net with csharp

Question

i have following three fields as follows.

Diesel cost    textbox1   combobox1

diesel rate     textbox2   combobox2

amount          textbox3  .

i want to validate when i run this suppose when user click combobox2 first want to show message  as first select combobox1 first how tovalidate.

please help me.

regards,

narasiman P