To make the platforms for your game search "platform sprite" or add a term in front like "graveyard platform sprite" or "cave platform sprite". Put the sprite through the background remover.
Upload it as a sprite in your game and hide it. Use the select tool and draw a box around the sprite you want to add toy your game. CTRL+C to copy it, switch to your ground sprite, and CTRL+V to paste it. You can move it where you want and also paste multiple times. Anything you put on the ground sprite is solid and your character can't go through it.
You need to create AT LEAST 6 levels for your character to get through. It should look like this when you are done. The ground should be the same, just add obstacles for your character to jump on or over on each level.