Click on Play and then on the number 1 (that is the level). Then click on the blue button at the bottom right of the screen to continue.
Your job is to use the slingshot to hurl the fruit at the monsters attacking your castle. To do so, press with the mouse button (or your finger) on the fruit in the sling and pull your hand backwards, thus creating tension in the sling and release to shoot him in the direction of your choice. Try to hit the monsters with as few shots as you need, because you can run out of hero fruit. If you run out, or just want to switch, click on the fruits standing to the left and right of the sling to change the fruit you're using to fire.
Each monster is a bit different, some stronger, some weaker. On the top left corner you will see a meter showing how many of the monsters for this level you have vanquished.
Once you've won some coins, you can use them to buy special items, such as a bomb that destroys all enemies on the screen, a health potion for your castle that restores some of the strength of your walls, a freeze ball that freezes all enemies and many more.