The simplest way is to copy the URL from your browser bar, and simply paste it into a post or your signature. The forum software will automatically change it into a link. That's what I'm doing here to create a link to this thread.
Select whatever portion of it you want to work as a link.
Then click the icon in the that looks like a planet with a paperclip above where you type.
A dialog box shows up for you to past your link into.
It will show up URL=link...., but when you post it, it looks like