user login

I know it is possible, I'm just not sure how, but I want a user to log into the site then if they enter information into a form the database not only stores the form info but also who entered it, so I would have a field called user and it would either get theinformation from a cookie,which I have already tried but am not having much luck with or is there a way of avoiding cookies altogether and using the database on it's own, which would suit me better. Thanks in Advance George