I have recently created a Ubuntu 12.04.2 64 bit virtual machine on VirtualBox, and I am not very used to Linux (I used Linux Mint for a few weeks some time ago), so please refer the full name of stuff, not just "the what-not-command".

The problem is I can't set the full resolution my computer supports (I think it is 1366 by 768),

I have found similar questions and tried most of the respective solutions, thy did not work.

If I type xrandr to the terminal I get:

xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 1024 x 768, maximum 1024 x 768
default connected 1024x768+0+0 0mm x 0mm
    1024x768       61.0* 
    800x600        61.0  
    640x480        60.0  

As you can see, the maximum is too low. And in the settings of the screen (I mean, with GUI) only 1024x768 and 800x600 appear. I don't remember exactly which answer of those questions, but it was one in the terminal (again, with xrandr) that made the resolution I wanted appear (although it gave an error when selected, not even changing to the 1366x768 resolution first and then back to 1024x768).