What do you mean Design websites? do you mean design it graphically for someone else to create the markup, or create the markup from someone else's graphics, or both? will it be static content (xhtml) or will it be powered/generated by server side scripting/application (php/asp/j2ee/coldfusion/perl)...
For graphics (if you are doing that part youself) i'd recommend fireworks if you can afford it (Adobe have a demo, and im sure you can find "creative" ways to extend the demo

). and for those who say "photo is teh most cool for imagez" read this:
http://www.adobe.com/products/fireworks/ said:
Accelerate web design and development with Adobe® Fireworks® CS3 software, the ideal tool for creating and optimizing images for the web and rapidly prototyping websites.
as for writing the markup, you have several good options on osx.
personally I like skEdit and TextMate, but recently I've moved to a product called Komodo IDE which is cross platform, as this is much better suited for writing PHP/Javascript/XML/SQL
for CSS, either skEdit or TextMate will work fine, or CSSEdit is great if you prefer something a little more GUI based.
I'm TOLD that the latest Dreamweaver no longer produces markup-soup, but I'm not going to put any weight behind that just yet.
If you are developing with PHP/MySQL, and done have a dedicated dev. server, I strongly suggest using MAMP locally. It is free, easy to install, and gives simple control to change between php4 and php5, as well as starting/stopping the Apache and MySQL servers.