You start up in graphical mode, correct? And then you want to switch to a text terminal?
I am guessing here.
But, it that is what you want to do (i. e. work in textmode) try this:
Edit /etc/inittab and change the
MAKE SURE YOU CORRECTLY SET IT TO 3. Any other values except 3 or 5 can make your system unusable!
What will now happen is that your system will come up in textmode. At the console login type your username and password as usual, exactly the same like you used for graphical login.
You should now be able to work in text mode to your heart's content. If you want graphical mode again, you should be able to get into it by just typing
Hope this helped!