Have you tried looking for a tutorial? I'm sure there are several on this site alone.
First, you need to create a form to upload with. It'll look something like:
<form enctype="multipart/form-data" method="post" action="upload.php">
<input type="file" name="userfile">
<input type="submit name="submit" value="Send!">
</form>
This form just displays a file upload box ("Browse..." in IE) and a "Send!" button (submit).
A PHP script ("upload.php" in this example) will then process the command:
$FILES["userfile"] is the file data. There are also these additional variables for use:
$FILES["userfile"]["type"] -- The mimetype of the file
$FILES["userfile"]["name"] -- The name of the file on the user's computer
$FILES["userfile"]["tmp_name"] -- The temporary directory and name of the file
$FILES["userfile"]["size"] -- The filesize (in bytes) of the file
$FILES["userfile"]["error"] -- The newest, any errors you may have received uploading
To move the file (actually upload it), you'll have to use either move_uploaded_file() or copy().
