You can find this answer very easy by using google.
I would do the following. Create on your desktop a directory website. In this directory create a directory called “css”, “js”, “img”. Create in the directory “website” index.html and enter this in the document:
<head>
<link rel="stylesheet" type="text/css" href="https://stackoverflow.com/questions/29539356/css/bootstrap.css">
<link rel="stylesheet" type="text/css" href="css/main.css">
</head>
Try making a container in your html file like:
<div class="container">
some text...
</div>
Use inspect element in firefox and you can see if your browser reads your css file.