sounds like you want your game to have different states, or like you called it "pages". so would be a good idea to create an in variable storing the actual state/page as int.

in draw you can use is to draw the different pages
for example

if(state==0)pregame();
if(state==0)page1();
if(state==0)page2();
if(state==0)gameover();


depending on how you gui should look like you can either program it yourself.
mainly by defining reagions on an image/ on screen and change the variable when clicking on them.
another way would be to make use of one of the GUI libraries available.

there is controlP5: http://www.sojamo.de/libraries/controlP5/
or Gui4P: http://www.lagers.org.uk/g4p/applets/g4p_showcase/index.html