In the June of 1914 Franz Ferdinand was assassinated by the Black Hand because he posed a threat too Serbian Independance
Germany declares war on Russia
Austria Hungary wanted to be involved in the investigation of the assassination in Serbia. Serbia refused which caused Austria to declare war on Serbia
Germany Declares War On France
After Austria had officially declared war on Serbia, Russia mobilized troops at Germany's border
Great Britain declares war on Germany
Germany officially declares war on Russia because of the Russian troops that are sitting at Germany's borders.
Germany declared war on France and because Russia and France were allied. So Germany decided to quickly attack France before the Russians could arrive.
The ports of Belgium are very close to the British coast so if the Germans got control of Belgium they would be a very large threat to Great Britain.