Posting links is different depending on what site you are putting it onto.
All of them start by copying the URL. This is the line of text that starts with http:// that is found in the address bar at the top of your page.
In some places, Fluther included, if you simply paste the URL it will be transformed into a clickable link.
If you want to wrap your words with a link like I just did there, you have to enclose the words you want in quote marks (”), then put a colon (:) then paste the URL. This is explained in the line of text right below this comment box I am typing in.
There are other ways of inserting links, such as UBB code (seen on bulletin boards) and HTML (seen all sorts of places). I won’t bog your brain down with those methods, but if you google “ubb insert link” or “html create link” you can find instructions for creating them.