Open a terminal (e.g Konsole) and cd to /etc and either:
type: su
type: [password]
type: chmod 777 inittab
You can then edit your inittab with a text editor. Remember to change your permissions back tho when you've finished (chmod 644 inittab)
...and that's it
*or*
type: su
type: [password]
open up a text editor, eg.
type: vi inittab
(edit the file (you may need to press the insert key to start, then esc once you've finished). Then ':wq' to write/save and quit)
or (easier) type: kwrite inittab
edit the file, save & close, and exit from root (ctrl+d or type 'exit')
[edit: when you update Mandrake 10 (if that's the distro your using) there a download that offers to change your inittab so that it defaults to runlevel 3 (it's one of the files you're prompted to 'inspect')]