I guess
this should be your starting point. Also take the time to read
the default content, it contains quite an extended introduction.
A very short intro:
Create a new template (Layout => Templates, click 'Add New Template') containing your html and put
in the code where you want the dynamic content (editable by admins) to be and
inside the section.
Then paste your stylesheet code into a new stylesheet (Layout => Stylesheets, click 'Add a Stylesheet'), attach the stylesheet to the template (Layout => Templates, click the CSS icon besides the template) and you can start creating pages that use your new template (Content => Pages, click 'Add New Content').
Normally you would use at least a {menu} tag in your template (to auto-generate a navigation menu) and probably
some more. Take a look at the default templates that come with the install for examples.
After that it's time to look at how modules work /how to include them in your pages. Each module has a help page that gives info on how to use it (Extensions => Modules, click on the module name or the help link on the same row). You can install new modules from the admin (Extensions => Module Manager).
Hope that helps,
Regards,
D