How do i prevent web form from refreshing after selecting value in combo box

I need some help. I not a prolific web developer, so any suggestions would be greatly appreciated. Basically I have a web form the is used to enter information into a works order management system. On the form itself I have a handful of textbox's and combo boxes of which I populate on page load. Two of the combo boxes are filtered upon selection of the previous combo box values. When I select a value form the first combo box it retrieves information from the database to load the second combo box, at the same time it refreshes the whole form. I don't want this to happen. I just need a quick simple bit of code to prevent the web page from refreshing upon the combo box value being selected. The only combo box event I am using is _SelectedIndexChanged, which passes the value to the method that connects to database and loads the next combo box in the sequence. I haven't attached any samle as i think it is a real easy thing to do for all the experts out there, but if required i can paste the relevants code snippits.

Thanks