It's a matter of using the bbcoding system ^^ Like using [ b ] (no spaces) to bold something and then closing it with a [ / b ] so it comes out like this.
In your case, you would be using the [ url ] tag. So what you want is going to look like:
[url=http://forum.jphip.com/index.php?topic=37323.0]It Started with a Hi[/url]
You're putting what the URL is in the first url tag, and then everything you write after that will become the link. Once you want to end it, you will finish it off with that [ /url ] tag to show that you don't want anything else after that to become a link.