There are some good articles on this, my favorites are:
Create WordPress Theme in 5 minutes
So you want to create WordPress themes huh?
That’s all well and good when you are hosting a WordPress blog on your own web server (or at least have full access to the WordPress files and scripts), but what about blogs hosted at WordPress.com, like this one at http://davebartrum.wordpress.com?
For a good explanation of WordPress.org and WordPress.com, see “WordPress.org vs WordPress.com“.
WordPress.com doesn’t allow you to modify the standard themes, or even to upload your own theme. Options appear to be:
1. You can edit the theme’s CSS Stylesheet, if you pay for the “Custom CSS Upgrade“.
2. You can create your own Theme and persuade WordPress.com to add it to their standards (I don’t know how practical/feasible this is, but probably your theme needs to be quite different to those already in there)
