Wordpress themes are like MyBB themes bit little difference there is, I've worked with wordpress before made few themes and released them for free even though they were mean't to be Premium lol.
Alternatively, you could use a theme creator like Artisteer.com
If you don't need anything too complex, but just want something original, this software combined with your own stock photos will create themes no one else has.
As long as you have a decent knowledge of HTML, CSS and PHP with some database skills you can create some pretty good themes.
With all the themes out there I am not certain why anyone anymore would be creating themes though. There are 10's of thousands of them available. Are you looking for creating something for yourself or the commercial market?
As above said you should have basics of PHP,HTML & CSS. You can always start with a starter theme like "Underscores" or download already approved theme from WordPress.org. Start playing with it and you will surely get the idea what's happening. For starting off with WP themes just get the basic structure of WP on codex as what files are mandatory and how WordPress handles different pages.
Due to the sheer numbers of Wordpress themes that are already out there that are good, it would be a lot easier to just find one that fits your needs as close as possible and then just modify it to fit your exact needs, Save a little time anyway.