Before I move my arm the brain sends signals - what causes the brain to send signals?
Action potential - I haven't read through the entire Wikipedia article so I can't vouch for its accuracy (not that my neurophysiology is good enough to critique it), but it's probably a good start.
Broadly speaking: the brain contains different regions that are each responsible for fairly specific tasks. In the case of picking up a glass of water we can, at a high level, describe the mechanism behind thirst. Your blood contains electrolytes in water; you can consider the electrolytes as being like cordial (reduce the water and you have a higher concentration of cordial / electrolytes). A sensing mechanism (beyond the scope of this answer) will trigger action potentials between the relevant areas of the brain (motor, memory, etc.) to result in the final movement.