Where did the Germans invade to start world war 1?

The question as posed does not really make sense. It was the Russian defense of Serbia against Austria-Hungary's invasion that started World War 1. The German declaration of war against Russia then triggered the alliance of the Triple Entente, expanding the reach of the conflict.

Once the Triple Entente was involved, the Germans wanted to invade France. But in order to do so and outflank the French forces on the German/French border, they had to invade the north of France, going first through Belgium. So Belgium was the first country to be invaded.