Hello greenexit,
I used the WP codex as a tutorial to figure out how to create new WP themes myself. But after a few disappointing results that were just too boring, blah and basic, I decided to take a free WP theme and modify it to my tastes. Check the license agreement on the theme you want, because sometimes even though they say it is free, it could have some restrictions. I try to use those that are without restrictions, so I can take it and almost transform it into a completely new theme. It helps to use the theme as a starting point, such as the menus, or the general layout, and style with CSS. Curious to see some examples of your work when you're done.

I modify a theme I'm already liking. Sometimes the theme author does something wacky in the .php that eats up some time for me, but I can't imagine it's ever been a tenth of what it would be to start from scratch.

Most wordpress themes are GPL. They can be modified to any extent. Makes it a lot easier than starting from scratch. Google wordress theme framework and take a look at the Justin Tadlock stuff. It makes it really easy to get started if you know a bit about WP.

Well i think you can modified a theme buy not remove the link at the bottom , hmm ya

Try here:
WPDesigner So you want to create WordPress themes huh?

they also have a downloadable version

Starting from scratch can be a painful process; however, if you can come up with your own design that works, you can make it into your own theme.

Just remember, if you're using someone else's theme, give them credit