Create a custom field for a file, and get the url of the file

This is giving me major headaches. I just want to have a custom field always appear on the add a post page that accepts a “browse for file or image” input. Then I want to write out the url to that file or image on my page. I can do this by using the image/media upload icons above the post area, but doing so adds extra code-p tags, etc that are screwing up my tight layout. All I want is the url.
Please help! Lately I can’t get any responses to my problems on the forum, hopefully someone can jump in here. As I see it, I’m missing something very basic.