There's a few things to know
1 Simple and easiest
Get the address in its entirety from the webpage you wish to send, say
http://www.acmecomputer.com.au/
If you include the http part, most email programs can interpret that as a web page address, activate the link automatically and the viewer can click and progress to the page. You might not see it before despatch...
2 Manually make the address a link
In Mail [Edit menu - Add Link...] you can activate a link manually and it can be any piece of text
So Acme Link is activated and will take you to Acme Computer without much further ado.
Copy the address from your webbrowser [on clipboard]. Select the text in your mail message, go to Edit - Add Link... - OK
Presto. Complete message, send.
3 Make ugly addresses look better!
You can disguise an elongated or ridiculous looking weblink to something less scary
AcmeItaly link
Note that AcmeItaly is located in Livorno, in English: Leghorn, obviously named after FogHorn Leghorn
Also
4 Sending webpage addresses from Safari
In Safari
Navigate to desired page
Under File - Mail link to this page, creates an empty Mail message with the link ready to go
Complete message and send
You can also send contents but this puts the contents of the webpage into a mail window at the other end, may not display properly.
Firefox -> File - Send Link - does much the same thing
