I think this guide I made awhile back might help you out a little bit.I am thinking about making an updated and advanced version of my guide using images, and such though.
Or you can highlight the word or phrase you want to turn into a link, click the Link button above the 'Post Your Reply' box... a window will pop up. In this window you type the desired URL (site-thingy-at-the-top-of-the-page)