First and foremost, The War Guilt clause of article 231 in the Versailles Treaty states that, "Germany is to be blamed for all the damages and losses Germany and all her allies caused following the antagonism of Germany and all her allies.
So, no one forced Germany to take responsibilities of what happened in WW1, the blame was shared between Germany and all her allies.
But as the main instigators, Germany should take the major part of the blame for starting WW1. Kaiser gave Austria the unconditional support in opposition to Serbia, in the heat of the July crisis, even with his knowledge of the fact that it would certainly lead to war with countries like Russia and her ally, France.
Moreover, Germany was the only country who had the capacity to turn a local misunderstanding into a global crisis that will begin with sweeping across the entire European nations. According to the Schlieffen plan, the announcement on August 1 as regards the German Army mobilization in line with the railway plan of the German Army meant war within 2 days against Belgium and France. Both the German high Army Authority and Kaiser were well informed of this plan. On the same August 1, Germany declared full war on Russia.
Lastly, the Germans needed a total European war which they were sure of winning quickly to start. This is obvious from the interactions that had gone on between Kaiser, his high army commanders, as well as his chancellor, which were all recorded and documented in the prestigious imperial archives.
So the blame of starting the First World War goes to Germany because they started it actually.