First, you need an additional attribute within the <form> tag:
<form name="formName" action="upload.php" method=POST enctype="multipart/form-data">
The enctype attribute will allow you to specify a file input tag within the form. You may also need a hidden attribute for specifying the maximum file size:
<input type="hidden" name="MAX_FILE_SIZE" value="100000">
This is the size of the file in bytes, not kilobytes. This is only a rough guideline, though - depending on how PHP is configured on the server can overrule this statement.
Finally, you will need your file input tag:
<input type="file" name="fileUpload" size=30>
When your form is submitted the variable $FILES["fileUpload"] will be used for the name of the file. This is split down into an array:
$FILES["fileUpload"]["name"] = original name of the file on the client machine.
$FILES["fileUpload"]["type"] = the MIME type of the file, if the browser provided this information.
$FILES["fileUpload"]["size"] = the actual size of the file, in bytes.
$_FILES["fileUpload"]["tmp_name"] = the temporary name of the file that is stored on the server.
The following link will give you more information on file uploading:
http://www.php.net/manual/en/features.file-upload.php