How do I fill out a form to be sent back to the sender that was part of an email?

What kind of "form" is it? A .pdf or .doc file?

Generally I would save the file somewhere (desktop or document folder). There is a save button for attachments in mail's header section. Then open the file with an appropriate app like Adobe Reader for .pdf files or Word or Pages for .doc files. Fill out the form and save. Then email the form back as an attachment.