Security Login and registration to system

[eluser]zend[/eluser]
Hi all I want to ask experienced programmers to advise me what is most important check when logging into the system and register to avoid hacking. I want to be safe so I would like to ask you a few tips....

I always do just this.
When a user goes to log into the system check if that user exists in the database if there has successfully logged into the system unless it returns FALSE .. But I think it is still weak and it is not safe.