Firstly, what grpahics driver (hardware) do you have? ATI, NVidea or other?
Ubuntu has one of the most intuitve xserver configuration out there (just my 2c first).
At the command prompt type
sudo dpkg-reconfigure xserver-xorg
Select the options that apply to you. I normally choose the all defaults, and at the driver scetion I choose VESA and select the resoltuions I know will work with VESA driver (in your case choose highest 1024x768, BUT if you have ATI or other card then u will need a prop driver to emulate that resolution..but that later). At the point where it asks you to autodetect display It will ask you twice, choose YES at the first, and NO at the second. It will ask you to overwrite the xorg.conf file, choose YES, do not worry it makes a backup of the old file (with the date and time as appendix).
Now you should come to a GUI login. If not then your resolutions were too high for the VESA, so go thru reconfigure as above and choose one less than 1024x768 res. You should be able to get into GUI now.
Try and find out what graphics card you have and we can take it from there?