Are you familiar with using a text editor, like vi or nano? There is a text file you want to edit: /etc/X11/XF86Config-4. You can edit it by typing in the following commands:
su (this lets you log in as root; it will ask for root password)
The basic way to use vi is to press "i" to enter "insert mode", so you can type in stuff. Then, you press <esc> to return to "command mode", so you can type in commands. The only commands you absolutely need is "ZZ" (two capital "Z"s). This command saves your changes and quits.
You want to go down the file until you find a list of screen resolutions on a bunch of lines. The first listed resolution by default is "800x600". Type in "1024x768" or whatever other resolution(s) you want before that one.
After making those changes, you can test them out by first logging out to the graphical login screen (gdm), and then pressing <ctrl><alt><backspace>. This keyboard shortcut restarts the X server. GDM should come up at the new higher resolution.