Making a theme in wordpress

Hi guys, really sorry if this is a stupid question but how do I go about designing a theme in wordpress, I understand the php part and css (Well I dont understand it but I understand I will need to understand it)
What I dont understand is how to make the design for it like the canvas of the website, is making a site in dreamweaver and then converting it to a wordpress theme a good idea or is it more efficient to build the theme using a local host and editing the php files?

If it is better to make it on a local host can someone explain to me how i would make the site graphical rather than just being a unattractive organised mess?

I’m sure you can tell I’m quite new to this, I can use WP easy but I feel I am very limited as I dont understand themes or plugins very well

Any help will be appreciated

Thanks