Prequisite

Geocoder Reference in Google Map JavaScript API v3

    http://code.google.com/apis/maps/documentation/javascript/reference.html#Geocoder

Google Maps Geocoding API(to know what attributes are return in result and which refers to what)

    http://code.google.com/apis/maps/documentation/geocoding/

autocomplete Jquery UI(1.8.12)

    http://jqueryui.com/demos/autocomplete/

Steps to use

1. Include Jquery js and Jquery-UI js and css in html code. For your convinience, you can use the one that I have added the in this project.
2. Include following line in html head

   This is needed to use google maps code

3. Include our geocode_autocomplete.js
4. Assigned class 'geocode_autocomplete' to input text box which you want to get auto filled based on user input.

Please refer working sample_geocode_usage.html for exact code.