We will need a bit more info on what your script is doing before anyone can say what the problem is.
Generally speaking though on your form handling file, temporarily do this:
and inspect the 'error' array - if it does not say 0 then there should be a big hint there.
To be honest, I found the best thing to study is the entire file uploading part of the manual, there are many good explanations and hints on how to debug this kind of operation. It'll take you a while but is well worth studying word for word before attempting this kind of operation (see common pitfalls especially)