Describe what you mean by "tool"?
Do you mean something that will take what you've designed and convert it for you? Unfortunately coding is a bit more involved than that.
You can try Adobe Dreamworks, Photoshop, and Fireworks as a good start. If you're on a Mac, there is a great development program called "Coda."
I generally design in photoshop, crop and export the pieces I need, and then code the website by hand.
If you have any other questions, let me know!