There's nothing special about Excel files. Just do an <input type="file"> in your HTML and have the user select it. Make sure that you define the form's encoding type to multipart. e.g.:
<form enctype="multipart/form-data" ... >
Due to security restrictions, the user MUST select the file manually -- you cannot specify a default value to point to somewhere on the user's disk. On the server side, you need to use something like Apache Commons FileUpload project to parse the multipart submission.
http://jakarta.apache.org/commons/fileupload/
Jon Emerson
http://blogs.adobe.com/jon.emerson/