I recently went to a museum that had a German fighter plane in it (not sure which one) and I noticed that the guns were behind the propellers, how did they shoot without hitting the propellers?
The short version is the synchronizer gear. Synchronizer gears linked the propeller shaft and mechanisms of the guns in such a way that the gun was blocked from firing at certain points along the rotation of the shaft. By aligning the synchronizer correctly, the gun could fire freely in the gaps between the propeller blades and still be prevented from firing when a propeller blade was in the line of fire.
Synchronizers took a fair amount of time to be developed, so several other methods were tried. Entente fighters for some time were reliant on either a gun mounted high enough to clear the propeller blade or pusher configurations that gave the gunner a clear field of fire. Roland Garros, a French pilot, would develop the first practical system for "safely" firing through a propeller using a system of steel wedges on the backs of the propeller blades to deflect bullets away from the propeller. While this system was more successful than one might expect - Garros is credited with the first ever downing of an aircraft firing through a tractor propeller - it reduced the effectiveness of the propeller and put undue stress on the engine.
Ultimately, Garros' development was quickly overshadowed by the introduction of Fokker Eindecker fighters equipped with synchronized guns. This allowed for a brief period of German air superiority over the front, although the impact was limited due to strict orders to avoid flying over Entente lines lest the aircraft and their firing mechanisms fall into enemy hands. Though the Entente would break the hold of the Fokkers primarily with pusher-propeller fighters like the F.E.2b and D.H.2, synchronizer gears would soon be developed and employed by the Entente as well.
The technology would be perfected by the end of the war, and through the interwar period synchronized machineguns would become the standard for fighters. During WW2, wing-mounted guns became commonplace (especially in American fighters), but synchronized machineguns and cannons would be the standard in most Axis and Soviet fighters through the end of the war.