
From the Apple menu: Select, System Preferences> Universal Access > Seeing
From here you can activate the zoom function which lets you zoom up to 40 times normal size (depending on version). There’s also the option to “Switch to White on Black” to make it easier to read text for example on a low contrasting background.
Note that the Universal access options will look slightly different depending on what version of Mac OS X you have but the core functions should be the same.