If you want the person to be able to attach a file that resides on thier local computer you would need to create an upload app and have the file uploaded to your webservers file system and then have the email app grab it from there and send it out. You would of course want to delete the file from your servers file system afterwards.
As far as setting up the browse button in asp.net you would use the HtmlInputFile control.
I would start here also go to www.asp.net and search the forums.
Then you would just need to wrap the upload and email together.
Good luck