In case you don't know how to follow the instructions that masonm gave, I'll expand a bit:
To change to a different directory, you use the cd command (change directory), e.g. "cd /var/log" (without the quotes). In your post, you said X told you the file was in /var. Are you sure this is correct and it wasn't /var/log? Also, the file will likely be called Xorg.0.log and not xorg.0.log. Case sensitivity is important and as such, the two names in the previous sentence are not the same to the shell.
As for reading the file, there are multiple ways to do this. You can use cat, which will just write all the contents of the file to the screen. Since the file may be quite long, this may not be the most useful because you won't be able to scroll. Another option is to use less, which will allow you to scroll using both the arrow keys and the page up/down keys. To just print the end part of the file to the screen, you can use tail. Edit: for all 3 commands, just write the command followed by the filename, e.g. "tail Xorg.0.log".