Assuming that you "sub-menu" is a Gui, you just turn it visible on click, like Slasher said.
You might have to adjust your Z order for the GUI with a higher number than your main menu (assuming it's a GUI as well), so that it's displayed in front of it.
You might have to adjust your Z order for the GUI with a higher number than your main menu (assuming it's a GUI as well), so that it's displayed in front of it.