well....... There is a Focus control on what is called the flyback transformer. That is the component that transforms your low voltage into a high voltage into you CRT.
The focus control is often a screwtype potentiometer (the type where you will need a screwdriver to turn it). And its often accessible from the outside. You should be able to see 2 or 3 small holes (about 3mm wide) on the bottom of your screen. Where one is the Focus Control and the other is Screen Control (Brightness)..... If you cant find these small holes you might have to open your monitor as mentioned above..... Wich can be really dangerous...
Inside the plastic cover of you monitor you will have a metal kind of cover, wich is used as a shield to reduce radiation. If youre lucky you might be able to have access to the controls through the metal shield.

I would dare to advice you to remove the plastic cover and search for the focus and screen control but I would strongly advise against removing the metal shield if you dont know what you are doing.