If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Sonic is in the garden swarming with the enemies. Throw an apple at flying ones and jump on the crawling ones to kill. Avoid the enemies poison and your own apples as they can hurt you too. You have 5 lives, sometimes you can catch life bonus. There are 20 levels and each level is harder than the previous one. To pass to the next level you should kill all enemies.
Left / Right Arrow Keys - Move.
Up Arrow Key - Jump.
Spacebar - Throw.