sipapu,
That's easy

First check that you have the Rich Text settings.
Go to Edit, Preferences. On the left side click on the + for Mail adn Newgroups ans then go down and click on formatting. Now look at the right there and check that you are using the HTML editor.
If you have not been using the HTML editor and check it you will also see that you have lots of added buttons and options for fonts, color, images etc.
Then all you do is copy the link into the email.
It will not show up as clickable till you send the email. So test it by sending it to yourself.
Now there are other ways also.
When you have a link you want to paste you right click and pick Insert new link anda box comes up.
You put your link in the Link To box.
The very top box is for what ever you want to have as the name for the link.
Like you can say go HERE to see this. All the see is the HERE but the link is there still.
You can type out you email this way and the go back to add the links by highlighting a word and right clicking and picking create link using selected.