updating my server

Hello,

since I'm using the directadmin control panel for my dedicated server "apt-get" is not an option.

which applications/modules need to be updated frequently?

That's a very general question. In a distro most packages are updated frequently, but not because of security problems.

OK, how do I know whats need to get updated? maybe I need to ask this at the directadmin forum?

For the control panel I guess yes. For the distro you can check Aptitude

or use

and this way only ubuntu is upgraded? DA has its own file locations for php, apache and some other programs

In that case you  should first check the DA documentation first

btw are these updates very important?

Not really. It is important only if there is a new security exploit.