you either choose a template similar to your website, or you take whatever template you are using and redo the graphics and the colours and css of that theme.
That is the only way I can describe it.. I liked the code, structure and SEO of the ragiels dream theme http://pribadi.or.id/diary/2005/06/06/ragiels-dream-theme-for-wp-15/
theme so I took it, stripped it, rearranged , tweaked here and there and added plugins.
and now it looks like
http://www.aleeya.net/
and
http://www.girlgeekette.net/
(Dont forget to give credit to the original author)
So, you can take it, modify it, and make it match what you have. or pay and have someone make a theme for you 😉