I would suggest using google often. When it comes to coding some of the answers you need you will have to figure out on your own, but that doesn’t mean there isn’t information to help along the way.
Have a read:
W3 Schools - CSS How To
One note is that the link to the stylesheet also depends on the origin. Say if it’s coming from a CDN(Content Delivery Network) you may see a link like this:
<link rel="stylesheet" type="text/css" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">
Whereas if your stylesheet was coming from your own hosting provider/server you would see something like:
<link rel="stylesheet" type="text/css" href="./mystylesheet.css">
As for terminating single tags I’ve found it’s more of a preference of convention thing. “<br>” and “<br/>” work just fine.
Do a bit of reading and then try to write the stylesheet link from scratch.