You didn't specify which distribution of Linux you're using. Is it RedHat 8? Mandrake 9? Slackware 11? (That last one doesn't exist yet, so you must be wrong. :P ) It helps to edit your user profile so we know and can give more specific help.
How to get to root text mode from a graphics screen?
Easiest way: hit Ctrl-Alt-F1.
This gets you to "console number 1", and you can log in as root. You can go to "console number 2", too, with Ctrl-Alt-F2. You can go to "console number 3", too, with Ctrl-Alt-F3. See a pattern developing here?
How do you get back? Well, keep going through consoles. Sooner or later (usually "console number 7"), instead of a text screen, you get a graphics screen, the one that you originally came from.
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Actually, when you don't have the graphics screen, you can use Alt-F2 instead of Ctrl-Alt-F2, etc. But with the graphics screen, Alt-F2 and Alt-F3 etc. already have other uses, so they made it Ctrl-Alt-F2, Ctrl-Alt-F3 etc. to switch between consoles.
If you don't want to leave the graphics display, you should look for a way to run "terminal" or "shell" or "console" or "konsole" (I'm guessing you use KDE). If you use the default install in Mandrake Linux 9.2, which is what I use, go to K-menu > Terminals > Konsole. There's a computer screen (get it? "console"?) with a seashell (get it? "shell"?)
You should get a text window within the graphics screen, with a prompt: you're already logged on as a normal user. From there, to get to root, type the command "su" (short for "switch user"). It will ask you for a password. Type in root's password, and you're in! When you're done, type the "exit" command to go back to being normal user.
If you want to boot up your computer running only text mode, and you know how to get a LILO prompt, I think you can type "linux runlevel=3" (someone check my syntax) and get text mode. If you don't know how to get a LILO prompt, then you should wait till you know Linux a bit more.
Peacedog's suggestion of editing the inittab file is rather advanced, and results in you *automatically* going to text mode every time you boot. I suspect that if you're at the level where that's what you want, and you know how to fiddle with inittab files, you probably wouldn't be posting your question in a newbie forum. (You can tell that Peacedog's pretty advanced, just from the fact that he's running Slackware.