populate dropdownlist with column name for search field

I need some help in populating the dropdownlist with column name. I have a gridview that  has 7 columns . I want  to have a textbox search and dropdownfield  where user can select column name and type a word in search box and click search so that text for that column name is being search and displayed in grid. can someone help me out how I can fetch just column names from DB and search ? 
...