I'm doing a project for school and need help

I am creating a "video game" for a school project and it's a simple platformer I need two things if possible I don't really know how this website works so any advice helps this project is due today and were presenting tomorrow. I need it to when I collect the last of a certain item particles fall from the ceiling to symbolize that you have "won" the game. I also need a background I'm not a big fan of the bland gray in the background. I don't really know how to do any of this and again any help is appericated.

For the background you can add a Tiled Background object or a Sprite with an image. When you collect an item you can add 1 to a global variable, when the global variable equals a value (total number of items you expect) then you could trigger a particle effect.

How do I do the global variable thing??