The very first time? That would be the Battle of Flers, part of the 1916 Somme campaign.

The Germans shot at the "tanks" a bit and then fell back to rear trenches, as was the german defensive doctrin. Only 9 of 49 Mark I's in the area that day made it to the first line of german trenches, mostly because they had technical difficulties and design flaws that physically stopped them from traversing or even reaching the battlefield. The Mark I also turned out to be very vulnerable to direct hits from mortars or artillery, which the Germans used to great effect. Especially the trench mortar proved effective against early tanks but it was necessary to heave it over the side of the trench and aim it directly, which was rather difficult to do under fire. Overall the tanks had a slight psychological effect (especially for the allied troops) but did not bring that much tactical value.

Despite what popular history "knows", tanks did not single-handedly make trenches obsolete. Early tanks didn't really achieve much at all. The first time tanks had real success was during the Battle of Cambrais at the very end of 1917, though that is limited to the first day of the battle, November 20th. And even on that day the german use of artillery pieces mauled them to such a degree that they did not manage to dislodge the german defenders for lack of infantry to do it. (The Germans then fell back during the night to prepare better defensive positions.) From the second day of the battle on, the Brits had lost the shock value of their unprecedented mass of close to 500(!) deployed tanks. In fact close to 200 of them were out of action after the first day of fighting alone. (Though most of them weren't destroyed by enemy fire.) The battle would last until December 7th, so you can figure out for yourself how the number of tanks in the fight would dwindle rapidly. The Germans were also reinforcing the area rapidly, which meant that the Brits only advanced very slowly and were eventually pushed back. Instead of being the great first victory for tanks and combined-arms tactics, what this battle really proved was that new infantry tactics and sudden massive artillery barrages were the way to go for now.

The german general staff wasn't impressed by what they saw. They allowed the development and introduction of armor-piercing rifles and ammunition, as well as special anti-tank weapons and tactics, e.g. the "geballte Ladung" ("amassed charge"), which was basically a bunch of grenades taped together. One type of tank was developed but only 20 were ever built. German forces used ~50 tanks in WW1, mostly captured british ones.

The value of WW1 tanks was massively overstated by people like Fuller, as well as due to the tendency of german infantry to run away from them - which, as I mentioned earlier, they were told to do even when only infantry attacked. Tank proponents also tended to give excuses like "had the terrain been more conducive to tanks..." to explain why their tanks did not achieve much.