In the big picture, Germany wanted a large empire and a large navy, which was incompatible with Britain's survival as a Great Power. This really didn't have anything to do with how the war directly started, but shaped a lot of the diplomacy around it. This doesn't assign responsibility either, as it would be just as reasonable to accuse Britain of holding Germany down as it would be to accuse Germany of trying to defeat Britain. No country has a divine right to any particular position in the world.
Further, the system of rigid alliances that characterized the start of WWI was a direct outgrowth of German diplomacy under Bismarck and later, if this can be considered a cause.
The German mobilization plan was partly to blame. Mobilization was the calling of reservists to the colors and the assembly of army formations in their positions to start the war. Once started, it would take days and almost certainly lead to war. The German plans were different in that their mobilization plans led into war, and they had only one. In event of war with Russia, the German plan was to overrun Belgium and attack Paris, and their mobilization plan would make that obvious. Since it was very dangerous to give one's enemy extra days to mobilize, allowing them to attack unprepared troops, there was a very strong reason to start mobilization on being informed of a neighbor's mobilization. To what extent this was responsible is also debateable. If it would normally have been possible for everybody to mobilize, stay within their borders, and slowly back down with diplomacy, the German plan would have prevented it. It did prevent Germany from trying to keep France out of the war diplomatically, although that would not have worked in any case. (The mobilization plan that ruled the context of diplomacy had been put together for military purposes, in typical German fashion the needs of a military campaign being more important than high-level diplomacy and strategy.)
The actual events that precipitated war were the assassination of the heir to the Austro-Hungarian throne by Serbian terrorists. The terrorists had some connections in the Serbian government, and we'll probably never know how high and how close. Germany sent Austria-Hungary an unconditional promise of support. Austria-Hungary sent a list of demands to Serbia. The Serbians managed to agree to almost all of them, but that wasn't enough for the Empire, and there came the first declaration of war. At this time, the President of France was visiting Russia, and presumably urging the Russians to start a war so France could regain territories lost to Germany in the 1870-1871 war (the President was very eager to get into a war with Germany under what looked like favorable terms).
Russia didn't want to see Serbia, something of a client state of theirs, overrun by Austria-Hungary, and ordered a mobilization since Austria-Hungary was mobilizing. Germany didn't want to see their major ally overrun by Russia, and had legitimate reason for concern since Russia was mobilizing next to them, and ordered mobilization. At that point, war was inevitable.
There are several points at which war could have been averted. If the Serbian government had been able and willing to control Serbian terrorists, the incident would never have happened. If Germany hadn't given the "blank check" to Austria-Hungary, they might not have gone to war, but this was an Austro-Hungarian decision in the end. It is almost certain that the President of France urged Russia into the war, and it's conceivable that without that the Russians wouldn't have mobilized, but again that was a Russian decision. Germany was probably forced into mobilization at that point.
So, there were a lot of actors and a lot of decisions that went into the start of WWI. Germany is clearly not soley responsible, but Germany was partially responsible.