When a new reservoir is built, where does the water come from to fill it for the first time?

Dams are usually (if not exclusively) built on rivers, often at the end of existing lakes. So rain comes down, goes through the various bits of the water cycle, gets into the river, and then gets trapped by the reservoir. Simple as that. And by the way, why are you only interested in the first time? Water is constantly going out of the reservoir into the water supply, so water needs to keep coming in. Pretty obviously: the first time is the same as the rest of the time!

It is pumped there from a water works. That's what a reservior is - a storage area from pumped, clean water.

The river it built to stop????

A resevoir isn't construscted HOPING it will FILL UP. and/or retain useable water. The thought is that a RIVER, might just flow into an ideal area, and/or runoff water from snow melts etc. Rev. Steven I doubt anyone in that frame of mind is waiting for the 40 days and nights of rain.

from heaven...rain

Rain fall and moorland sheep.

There's a tap, like on your bath, but only cold.

From the river that was dammed to create the reservoir in the first place.

rainwater.