From the main menu (if running X Windows and the default window manager):
System > Administration > Display
If your display hardware is correctly recognised, then you will have all possible (safe) resolutions in the popup list.
In addition to that you can manually adjust your X configuration to get other resolutions. No quick tips for that: you need to know what you are doing and what your display is capable of. You will have to read all the boring stuff regarding Xorg configuration and also check the technical description manual of your display for correct sync values.