You change the run level:
www.debianadmin.com/...buntu-linux-run-levels.html

But in Ubuntu 9.10(Karmic Koala) I can't find /etc/inittab.
I tried create one to change the runlevel but no success.

Ubuntu uses Upstart and the actual file that is changed is not inttab but *
/etc/init/rc-sysinit.conf.*

I think that you change it to run level 3. It is set for run level 2 by
default. I hope this is what you need.