Unity 2D GUI screen change help.

Hello. I'm trying to do a school project which behaves like a game. It's like an interactive tour of certain city. Say it starts up on a main screen and there is a button which says "Start". What do I need to do in order for me to press the button and go to another screen with more options(buttons) to be pressed?

I am also looking to have another section which behaves like an image viewer. Just stay on one screen having two buttons (to the left and to the right) and pressing them would swipe images to look at all of them.

Any help would be really appreciated.