you may want to look into
JQuery to help get the job done. it makes working with js much simpler. it's not oriented around widgets like yui, it's just a light weight lib that makes it super easy to work with DOM elements, effects, and ajax.
although it doesn't have an autocomplete built in, there are plugins for it which may help you. (
jQuery Autocomplete )
i don't have the time to look into giving you really good info on how to make one from scratch. i've never had to do that.