Instead of writing a tutorial on how to get you stated in the world of writing a WordPress theme from scratch, I will just give you a set of links to great material you can use.

Theme Development – WordPress Codex [http://codex.wordpress.org/Theme_Development]

You will become familiar with the basis and structure of a theme

http://www.w3schools.com/

Great source of documentation for HTML, CSS, jQuery and more

PHP Documentation [http://us3.php.net/docs.php]

The source of knowledge if you are still not very savvy with PHP

WordPress Documentation [http://codex.wordpress.org/]

Similar to the PHP Documentation, this source will answer all your questions related with the APIs that fire up WordPress

/wp-content/themes/twentyeleven (and twentyten)

There is no better documentation than the code, in this case such source of information is the theme than ships together with WordPress and allows you to navigate and find out how concepts and APIs work in a very good sample put together

So hopefully with this compilation of information you will feel more confident to start working on your own WordPress theme and you will be able to let our creativity drive you towards a great user experience for your users.