Need code in C# or ajax, javascript

hi i'm new to C#, please help.I need  C# code for I have Dropdown list some file names are listed in dropdownlist when page loaded from database.Beside that there is a textbox, when i enter "Ac" in text box, all files  in Dropdownlist with starting latter "Ac" are should be display or sorted in same  dropdownlist please help me..........

example     onload Dropdownlist show file name : sddd
                                                                  gdgf
                                                                  sffgf
                                                                  sgeg
                  when i enter "s" in below textbox ,the above Drownlist  ll shown
                                                                   sddd
                                                                   sffge
                                                                   sgeg


thanks in advance