In the broadest sense, Germany didn't really have any territorial ambitions at the outset of World War One, outside of some diehard Pan-German nationalists. That is not to say it wouldn't have grabbed land in the case of a victory - the Low Countries, parts of France, Poland, some parts of Russia, and land for the Austro-Hungarian Empire in the Balkans all would have been on the table - only to say Germany was not planning on it, and certainly did not start the war(if indeed, it did) with them in mind.

Austria-Hungary and Russia were the ones with territorial ambitions at the start of the war. Russia to support Pan-Slavism, grab the Balkans, contain the Central Powers, keep the reactionary Tsarist regime stable, and seize Constantinople. Austria-Hungary's designs in the Balkans were threatened by Russia's advances there, as well as Austria-Hungary's government, and the multi-ethnic empire itself, from nationalism and liberalism.

Germany, at the outset of the war, was desperately trying to break out of the British-French-Russian encirclement it found itself in. Since its existence as a nation - and much before that, though how German it was is up to debate - Germany was terrified of an encirclement by hostile powers: as powerful as Germany was, it did not want to fight a two front war against three Great Powers. Any territory which broke the Entente would have been acceptable to Germany. Add to that concerns that Germany's only major ally, Austria-Hungary, was on the verge of breakup, and you have a very very nervous Germany. When the July Crisis came, the German government backed Austria-Hungary to the hilt, out of fear that there would never be another chance.

Most of this was taken from Brendan Simms's Europe: The Struggle for Supremacy

Well, Germany had just recently been united after the Franco-Prussian war of 1871 so quite a young nation. In 1888 Kaiser Wilhelm II rose to the throne at the age of 29 - young, ambitious and impetuous, making all sorts of important decisions on the spot without consulting his ministers. Almost immediately he started to transform the country's foreign policy to suit his greater plans, dismissing Chancellor Bismarck's Realpolitik and its concentration on a secure German Empire in favour of his Weltpolitik.
He desired an Empire that was not just sitting safely at home in middle Europe, he aspired to be like Britain, a global superpower with colonies all over the world to fuel industry and trade and a navy strong enough to secure the seas, Germany's "own place in the sun".

Of course that wasn't exactly an easy goal, considering Germany had only been united in 1871 while everyone else had been running around, planting flags left and right for centuries so Germany naturally caused a lot of fuss disputing borders with other colonial powers and relentless expansion of naval forces to rival England, despite having very little effective coastline in comparison.

Then France, being still very much annoyed at losing Alsace-Lorraine in the Franco-Prussian War and the Kaiser's new international meddling, allied with Russia in the Franco-Russian Alliance of 1892 to secure both German borders against possible aggressive expansion, which was eventually joined by Britain in 1904, not the least because of Germany's relentless enlarging of its fleet to rival the British Navy. So Germany certainly had good reasons to worry about that and try to avoid a two-front war with an Army only meagerly bigger than France's and a Russia that was doing huge leaps towards modernising their Army to a point that would have made it nearly impossible to beat, which had both Kaisers equally worried.

So concerning how much the Germans were planning on taking the answer is certainly all they could get, if only to show they could. For specific land gains I am not sure if there were many plans, but then again I am not a historian.
The only thing I know of there would be Fritz Fischer's Germany's Aims in the First World War (1967), where he talks about German Chancellor Theobald von Bethmann-Hollweg's war ambitions, his support of the ethical cleansing and germanisation of Poland and his Septemberprogramm, goals of specific land gains from France and Belgium, vassalisation of Luxembourg and possibly the rest of Belgium, getting Holland "into a closer relationship" and a sort of proto-EU under German rule. Although I understand Fischer's interpretation of taking this unofficial late-1914 draft and calling it a war reason is somewhat controversial.

Bethmann's programme outline: Security for the German Reich in the west and east for all imaginable time, Russia thrust back as far as possible and her domination over non-Russians broken, France to be weakened so as to make her revival as a great power impossible for all time, minimise the number of non-Germans absorbed into the Reich, a belt of colonial territory in Central Africa from coast to coast, and the annexations of Luxemburg, Liege, Antwerp, the Briey iron ore field in France, the western Vosges mountains, and perhaps some of the Channel coast. France was to be weakened by indemnities and treaties, Belgium was to be a vassal state.

Fritz Fischer's Germany's Aims in the First World War is the seminal study in this area.

The Germans sought an offensive war but they did not have territorial ambitions. They launched the attack on France because their backs were against the wall. The Dual Alliance between France and Russia was more than enough to crush Germany on paper so when conflict loomed in July, the decision makers in Berlin turned to their strategists who had been preparing for just such an eventuality for over a decade. The Schlieffen Plan did not represent a German desire to conquer Belgium or France, nor did Germany have vast designs in Eastern Europe until after vast amounts of lives and treasure were spent there fighting the Russians. It was simply seen as the one chance Germany had to win the war. People all around the continent became increasingly convinced of the likelihood of war that summer, so the plan was implemented.

German war aims during WWI are quite an interesting topic. If they had stuck with the defensive aims of the Schlieffen Plan (maintenance of their borders in the face of overwhelming force), then they might not have faced retribution at Versailles. But as more and more men died, it was only natural to expect benefits in proportion to the cost.

Holger Herwig, The First World War: Germany and Austria-Hungary 1914-1918

It's just sad, that mostly history here is something that is follwing the paths of the politicians.

• The german government planned before the war the domination of the middle of Europe.

• The german government worked since 1911 active to get into a war

• In 1912 the german government changed the direction for a forced build of the army to focus on the axis Russia-France

• The conflict with Russia was a calculated one to mobilize all parts of the citizen. The intention was to show Russia as an agressor which worked that time when even the sozialdemocrats voted for the war. The opposition in the socialdemocratic party was excluded.

• The german government motivated Austria to start the war.

• Not only the army and politicians voted for the war, but leaders of the industry too.

Source: Bernd F. Schulte: Deutsche Policy of Pretention - Der Abstieg eines Kriegerstaates 1871–1914. Books on Demand, Norderstedt 2009 * Hamburger Studien zu Geschichte und Zeitgeschehen. Reihe II. Band 1, ISBN 978-3837022513

One thing to keep in mind is that Germany did not start World War 1. World War 1 started with the Serbians, Austria-Hungary, and Russia. . .and then escalated from there via the web of alliances that had been established over the last few decades.