Open a terminal window and type more /etc/inittab. (do this as root -- you can use su - to switch if you're on as a regular user). Look to see if you have a line that says id:3:initdefault: or if it says id:5:initdefault:. If it's the first, then you are ok. If it's the 2nd, it needs to be changed to have a 3 instead of the 5. That line indicates what run level you want to boot into. Usually it's set to 3 or 5. NEVER set it to 0 or 6.

To edit the file you'll need to be root. Use one of your fav editors and change the line to have the 3. Reboot and it should ask in text mode for your user name and password. When you log in as a regular user, you can then type startx to start into the GUI interface. I've always recommended to students to always use runlevel 3 as the default rather than the GUI because if you have video issues you can resolve it faster.

Let me know if any of this isn't clear.