How do you get the Laplace transform of a square wave?

I am having trouble figuring out how to determine the laplace transform for a square wave. The square starts at t(0) and ends at t(3) with an amplitude of 2. I have been all ove rhte internet tonight and haven't found anything that has been of help. Any help would be appreciated I am completly lost on the subject I know it has something to with the Heaviside function but the instructor hasn't covered that.