How can I make my camera stop blurring my pictures?

I am attempting to do a project for school, but so far have had no luck using the camera I need for the project. It is a Canon Powershot S5 IS. It is my Dad's, and I don't have the technical knowledge to understand all the different facets of this camera. I've looked up manuals for it, looked at other people's solutions for blurry pictures, and tried adjusting everything that make sense, to me, to adjust, but all with no success. The subject of my shot is not very close, in fact I moved the camera back because I know sometimes you can be too close, but that didn't help. The camera is on a tripod, firmly mounted, but still the camera keep telling me it's shaking. I don't understand what is happening, why my pictures continue to blur, but I really need some kind of help. Even when the camera seems to have adjusted, I go to take the picture and it comes out blurry. Please, any helpful advice?