Do you have hosting on a server? You don't need anything but WordPress and an FTP client to install it on the typical web server. You just need a web hosting account that meets the system requirements which are very basic and should be met by most hosts.
Here are the instructions for installing WordPress, it should take about 5 minutes :)
The purpose of MAMP(Mac, Apache, MySQL, & PHP) is to upgrade your own computer to meet the requirements of WordPress. This way your computer is able to act as a web server, power databases, and render PHP code.
Here are the instructions on setting up MAMP if you didn't find them. Essentially, it shows you how to set up your Mac as a server, and then install WordPress onto your computer. But this means you must set up a whole server, installing WordPress right onto an existing web server live on the Internet is much easier!
You don't have to use it as is, you can edit the themes, but you can't edit the theme with a WYSIWYG editor like Dreamweaver. In WordPress, themes are modular, broken up into many different files.
If you unpack a theme and look at the file names, you can see what I mean, the page is spread across separate files for the header, footer, and columns. WordPress then combines them on the fly as needed.
So you basically have to edit the code by hand. You can create a design in a WYSIWYG editor as HTML and CSS and then convert them into a WordPress theme. I have created themes this way. But it can get complicated. Here is a little info on themes.
The best way to do it just starting off, might be to get a theme that looks closest to your existing site and just start editing the CSS a bit to match. Start by changing the color values and swapping out a few images, and it will all make sense eventually!