how can I use jquery autocomplete in drupal 7
Yes, jQuery UI is available in drupal core.

To include libraries like that you need to call the drupal_add_library() function to load the required js &amp; css.

For exmaple:

drupal_add_library('system', 'drupal.autocomplete');


So where you are adding your javascript, add that first.

For example:

// Add the jQuery UI autocomplete library.
drupal_add_library('system', 'drupal.autocomplete');
// Load the custom js for this module.
drupal_add_js(drupal_get_path('module', 'MODULE_NAME') . '/js/SCRIPT_FILE.js');


The caps lock parts need to be changed to suit your module.

As for where you would put this, I can't say unless I know where you are putting your code. It could be in your theme or in a custom module.

If it is in your theme then the drupal_add_js() part would be drupal_add_js(drupal_get_path('theme', 'THEME_NAME') . '/js/SCRIPT_FILE.js');

I'm also assuming your scripts are in a js sub-directory.

You also need to make sure that any custom js you have is inside a drupal behavior (or $(document).ready() at the very least). Otherwise your javascript will run to early in the page load before the text field even is on the page, so it will do nothing.

It will be like this:

(function ($) {
  Drupal.behaviors.yourThemeAutocomplete = {
    attach: function (context, settings) {
      // Your code goes here.
    }
  };
})(jQuery);


For more info see https://drupal.org/node/756722#behaviors (the whole page is useful info)