In a nutshell you can't!
Whilst it's relatively simple to get text into excel from the web using a web query it's left easy to import it.
If you have control of your website then the task is made easier with using something like ASP or PHP to pass information or upload files but you can't copy from excel straight to the net.
Can you be a bit more specific, do you have control of the website in question ? What language do you use, ASP or PHP or just straight HTML ?