How do you take screen shots on a Mac OS X 10.5.8?

I was wondering because i really want to take screen shots on my computer and most people say to press Shift+Command+3 or 4 but it doesn't work, and i don't want to go through the hassle of downloading a program,so does anyone know someway i can take screen shots on a Mac OS X 10.5.8 version by using a keyboard code or by another way thats already on my computer? Please if you can help in any way then please answer this question, thank you!