When you install a launcher, you have to first tell it to replace your current one. It doesn't take much time to learn the ropes, really.
After you install the launcher, it should immediately ask you to select which launcher you want to, well, launch, when pressing the Home Button. If it doesn't, just try going into the Application Manager, find the launcher you're using (if it's the stock one, like Samsung's TouchWiz, it will be under that name, TouchWiz) and select "Clear Defaults". This should prompt the list to re-appear.
Once you select Nova as your launcher, you can pretty much customize it to your liking. Are there any specific questions you have on how to use it? We can help.