I don't know too much about computers. How would I send a music video from the internet to my girlfriends email? Can someone please help me ?

It's all a matter of size as e-mail systems often have a restriction on attachments.
Easiest way would be to send her a link, assuming it's freely available online, otherwise there's potential copyright issues.

If you're referring to something freely hosted on the internet, such as YouTube, then you simply need to send them a link to view the page in question. This can be done by copying the url (usually the http://www address at the top of the screen), and then pasting into the email you're sending. Otherwise, curlyben has the answer, size restrictions are a matter of which email company you use. To circumvent these restrictions, I use companies like adrive.com to host files publicly and then share a url as above mentioned.