I suppose that you are opening the tap when the nozzle is already placed vertically above the ground.
Let's take the upwards position as the positive direction.
The initial speed of the water is positive,
The acceleration due to gravity is negative,
The displacement of the water is negative (ie, final position is lower than initial position)
Those settled, you can use:
Where s is the displacement,
u the initial velocity of the water,
t the time taken to cover that displacement,
a the acceleration the water undergoes.
So, we get:
Solve for u.
I hope it helps!
Concerning the equations, there are a couple that you need to know: