$this->load->library('upload');
$this->upload->initialize(array ('upload_path' => './yuklemeler/'));
print_r($this->upload->do_upload('inputun_namei') ? $this->upload->data() : explode(',', trim(trim($this->upload->display_errors(' ', ',')), ',')));

bu sekilde kullanabilirsin en basitinden.

tabiki form alaninda enctype="multipart/form-data" demeyi unutma

<form method="post" action="/upload/yapilacak/controller/ve/method/falan" enctype="multipart/form-data">

<input type="file" name="inputun_namei">

</form>