Hm...
OK. I feel you should have been posting in newbies..
You have no experience with PhP, do you?
OK.
PHp lives on the webserver. However, the upload box 'lives' on the clients computer (The user). So.. your script runs before the user even sees the upload box.
You need to split this up, say in 2 pages. On with the form, which posts the file to the next.
In the processing page, you need to get the filename from the posted file array
However, this is a bit more complex then how you think it goes. I suggest you read this
first before you proceed.