Designing web sites can be tricky at first. But when you get the hang of it there's nothing to it!
A very important idea is to separate style from content. These are the two main aspects of a web site. This web site explains what goes where.
Here is the basic structure of a HTML5 website...
Here we put the style of the website
</head>Here we put the content of the website
</body>It is a good idea to have all of your pages look simlar. The best websites are instantly recognisable by their look. As the appearance is controlled in the <head> section of each page, to make them all the same they should all have the same <style>. This can be a pain to update, so a good idea is to have the style in a separate file. This way we only need to update one thing and the whole website changes.