I wish to produce a special php and javascript page, however i would like it to use my Wordpress theme that's already installed. How to achieve that?

Thanks

This can be done by such as the header and footer from wordpress, and putting your personal code in the centre. Similar to this:

include $_SERVER['DOCUMENT_ROOT']."/wp-blog-header.php";
include $_SERVER['DOCUMENT_ROOT']."/wp-content/themes/[YOUR THEME]/header.php";
// your code goes here
include $_SERVER['DOCUMENT_ROOT']."/wp-content/themes/[YOUR THEME]/footer.php";

I've not examined this on more recent versions of wordpress, but previously which has labored. You may also range from the wpconfig file if you wish to apply certain of individuals variables, like the database connection information.

Hope this can help.

See Integrating WordPress with Your Website « WordPress Codex regarding how to pull the header along with other includes right into a static php page/file.

  <?php
    define('WP_USE_THEMES', FALSE);
    require('./wp-blog-header.php');

    get_header();

    query_posts('showposts=1');

     get_footer();
    ?>