In my combo box i have kannada words like

karnataka
kerala
mandya
mysore
kannada

something like this ,

On key press event of combo box if ka (kannada alphabet is pressed)
it should list the karnataka or whichever matches the word
how to do this

is the items stored in database ?

yes boss all words stored in database
and one more thing on page load they appear in the combo box

but what i need is autocomplete feature.

like if i press ka it should show kannada or whichever matches

Select your combobox, in the properties window scroll down to 'sorted' and change the property value to "true". This will load the data alphabetically. When you add any text to the combo box AFTER you selected it, it will automatically select text available in there in that alphabet letter, i.e. by selecting k, the combo will scroll down to "k".