The goals of all the Great Powers were varied and changed as the war dragged on, but Germany had fairly specific goals at the beginning of the war. We can look to Bethmann-Hollweg's "Septemberprogramm," which listed the goals Germany had if they were to win (and Germany was pretty confident). The main goals were to create a "Mitteleuropa," or a Central European economic zone dominated by Germany. Perhaps most importantly was their imperial desires for Africa. They wanted to create a "Mittelafrika" carved out of the defeated powers and brought into the fold of the German Empire. I'm typing on mobile, so I'm sorry I can't give a more nuanced answer to this great questions. Here is the (decent) Wikipedia page listing the goals of the September Program: http://en.m.wikipedia.org/wiki/Septemberprogramm

It should also be noted I'm basing this off the top of my head from my memory of the Fischer argument. I know it is a bit dated in the historiography, but I loved Griff nach der Weltmacht: Die Kriegzielpolitik des kaiserlichen Deutschland 1914–1918 (typically translated into the less offensive Germany's Aims in the First World War). Unfortunately, I'm not home so I don't have my copy handy, thus I cannot list the page numbers discussing the September Program. I hope that does not break the rules.

Germany perceived itself as encircled by the triple entente at the start of the war. And consequently Wilhelm II did emphasize that the war was defensive^1

A momentous hour has struck for Germany. Envious rivals everywhere force us to legitimate defense. The sword has been forced into our hands. I hope that in the event that my efforts to the very last moment do not succeed in bringing our opponents to reason and in preserving peace, we may use the sword, with the help of God, so that we may sheathe it again with honor. War will demand enormous sacrifices by the German people, but we shall show the enemy what it means to attack Germany. And so I commend you to God. Go forth into the churches, kneel down before God, and implore his help for our brave army.

Wilhelm II, Speech from the Balcony of the Royal Palace, Berlin, July 31, 1914

However after the initial successes at the western front, quite a few proposals for war aims were either published or send to the German government, ranging from proposals to "free the non Russian people of eastern Europe" and "military supremacy" over Belgium and northern France, by the Center^2 politician Matthias Erzberger, to predecessors of the Lebensraum policy of the Nazis,^3 by the chairman of the Pan-German League Heinrich Claß. In this context the chancellor Bethmann Hollweg did publish his "Septemberprogramm, which was labeled as preliminary.

On the other side of the political spectrum the social democrats (SPD) insisted that WWI was a purely defensive war, and consequently were critical of proclaiming war aims, while in 1915 the right started to fear a "spoiled peace" in which Germany would not gain enough. And to keep the Burgfrieden ( lit. "Castle peace", the notion that political struggle should rest during the war.) Bethmann Hollweg was rather ambiguous about definite war aims.

I think it is save to say that Germany wanted at least break the encirclement and remove France and Russia as a military threat, which would result in German hegemony of continental Europe. Going a bit further it is easy to argue that hegemony was the goal of the war, especially considering Wilhelm´s sympathy for German nationalists. On the other hand Fritz Fischer viewed the Septemberprogramm as a declaration of government policy, and consequently claimed that Germany planned and started WWI to annex parts of Belgium and France and construct a colonial empire in eastern Europe.

Source: Volker Ullrich, Die Nervöse Grossmacht 1871 - 1918, 2nd Edition 2013. (I am not sure if there is a English translation)

^1 The aggressor giving a public speech on the eve of war is of course not a unbiased source. But is shows how he wanted to sell the war to the German public.

^2 Center is the political party of the German Catholics. It is probably least misleading to think about them as moderate conservatives.

^3 That is ethnic cleansing and consequently resettlement by ethnic Germans.