How do I decode the data in my Form?

The normal format for data in HTTP requests is URLencoded.   All Form data
is encoded in a string, of the form
 param1=value1&param2=value2&...paramn=valuen
Many non-alphanumeric characters are "escaped" in the encoding:
the character whose hexadecimal number is "XY" will be represented by
the character string "%XY".
Decoding this string is a fundamental function of every CGI library.
Another format is "multipart/form-data", also known as "file upload".
You will get this from the HTML markup
<form method="POST" enctype="multipart/form-data">
(but note you must accept URLencoded input in any case, since not all
browsers support multipart forms).
Most(?) CGI libraries will handle this transparently.