Solarus's basic interface works like this: on the left side is a bar with the different elements that you can use, like tiles, sounds, music, maps, etc. Just double-click any of these to bring it to the center of the screen.
From the main part of the working window, you can edit all of the elements that you want to include. So you can move different pieces around the scene, create treasure chests with items inside, place enemies here and there, and basically do whatever you want.
It's important that, before you start working, you first import the game module that comes with your download of Solarus. That will give you many more materials to use while you work.
Solarus is a simple Zelda style video game creation tool that easily competes with other tools like the classic RPG Maker and the more complex Game Maker.