When you start, only a vertex is available for you to use, but as you gain money, more and more can be bought, which are better the more expensive they are. Use the mouse to click on the two arrows at the bottom to point towards one or the other direction, and click on the sword buttons in order to shoot, and attack. The goal is to attack and defeat all enemies that are going to come before you before they manage to hit you first and deplete your health bar so that you ultimately lose. In the end, we can only wish you good luck!