CodeIgniters session library - how to?

[eluser]v33s[/eluser]
Hello all,
I am new in CodeIgniter and I have a question about CodeIgniters Sessions. Can someone explain me how to start write simple admin login page area? I tried most of complete Authorization Systems but I don`t understand how to implement them in my code.

Ok. I have my `admins` MySQL table with -id-name-md5password- columns.

On first place in my Contructor, i have that code:

Code:

$this->load->library(array('session', 'validation'));

I would like to check the form values with the database values, if it is correct then make a session.

I can extract user from database, but how to make a session, check it if admin is now logged in???

I found in User Guide how to make a session:

Code:

$newdata = array(
                   'username'  => 'johndoe',
                   'email'     => '[email protected]',
                   'logged_in' => TRUE
               );

$this->session->set_userdata($newdata);

And now, what I must change in this array? Replace username with this from database?
How to check this session if it is correct?

Sorry for my English