Any good ressources for beginners or can I use…

Actually you can in fact use DreamWeaver to edit or create a stylesheet, or you can use a simple text application such as Notepad which is what I use. When using a text editor, simply save using the css file extension, or change the txt file extension to css. When using DreamWeaver, simply open a stylesheet, or choose a new stylesheet. If you are going to use it with WordPress, do not use DreamWeaver to link the stylesheet to the index page, unless you have already assigned a site folder. Then you can play with the attribute variables such as color values to get a feel of how the stylesheet works. It actually might be better to first experiment with stylesheets with an html page, but the choice is yours.
Some sources of information, but not necessarily all for beginners, are:
http://www.csszengarden.com/
http://www.htmldog.com/
http://www.echoecho.com/css.htm
http://www.w3.org/Style/CSS/learning