There are many ways to cut the mustard on this one. One easy way to do it is to download Ubuntu Tweak, found here. Once you install and run that, you will see tabs on the left. Click on "Window Manager Settings". There you can rearrange the order of the buttons just by dragging and dropping.

Another way is to open a terminal (Applications -> Accessories -> Terminal) and run the following command:

$ gconftool-2 --type string --set /apps/metacity/general/button_layout "menu:minimize,maximize,close"