I'd recommend notepad if you're comfortable with CSS, if you don't know too much about CSS then I'd use Dreamweaver as it has most of the CSS definitions built.
In the <head> of your HTML page add the following line of code
<link href="style.css" rel="stylesheet" type="text/css">
Obviously changing "style.css" to whatever you've called your stylesheet and linking correctly to it, the above code assumes it is in the same directory as your HTML page.
If you need any more help PM me or I'll keep replying to this post, I don't mind
