You can remove underlined link on HTML, by using inline style like below. It's so familiar.
<a style="text-decoration:none" href="link.html">
Not Underlined Link</A>
Normal Underlined Link</A>
Or if you want to put code onto internal CSS, type like below between <head> and </head>.
Internal CSS is Cascade Style-Sheet code in the same page that showed. If you would like to put it in External CSS that put on another directory, follow these method.
Make new CSS file and type the code below.
Save it as anyname with ".css" at the end of the file name. It will become as file extension. For example, you save it as "anyname", you must write the file name as "anyname.css"
Then follow the second step.
<link rel=stylesheet type="text/css" href="directory1/directory2/anyname.css">
directory1 and directory2 are the examples for directory root, you must make it same as where you put anyname.css file inside the directory of server.