Keyboard problem

I have got a mini mac and bought a generic white usb keyboard. After about an hour of my machine being on, when i type in a browser i get two or three letters for every key i press. It only works on some keys...but i cant work it out. Do you have to use a mac keyboard?

You don't have to use a mac keyboard but it sounds like the one you have is a piece of crap, take it back to get a different one.

You might want to check your keyboard repeat rate.
Go to System Preferences>Keyboard & Mouse>Keyboard and check the slider. You might have to move it to a slower repeat rate.