What weapons did they use for the 1st time in World War 1?

The airplane, the tank, poison gas, the machine gun,

I think the machine gun pre dates WW1, but yes, it's effectiveness was a major factor in the number of men killed.

The first yperiet (mustard gas, chloride gas -1,1-thiobis(2-chloorethane)) - attack took place on April 22, 1915 in Ypres, Belgium. The gas was invented by the German chemist Fritz Haber. His wife, who didn't agree with the use of deadly gasses on the battle ground committed suicide when the first attack with yperite took place under supervision of her husband.

The first tank (British) type MARK I was introduced on the battlefield September 15 1916. 46 were used, 32 reached the frontline.

A curiosty: The first wristwatches for men, called 'trench watches' were worn by British officers in the trenches.