Yeah I get what you mean now. The thing is though, you're actually asking about two different things.
On most forums (this one included) you use BBcode to make a clickable link. To make a clickable link in an email you use HTML.
This is a clickable link in BBcode.
[url=www.google.com]Google[/url]
And the result:
GoogleYou can underline it like a HTML link by adding another tag.
[url=www.google.com][u]Google[/u][/url]
GoogleThis is the same clickable link in HTML.
<a href="http://www.google.com">Google</a>
Can't show the result on this one. HTML is disabled on the forum.
Oh and FYI, it is important to remember the http:// part when making a link in HTML... else it won't work correctly.