Brief your question little bit, if you want to measure water flow you need to learn working principle of solenoid valve and sensor unit if you are measuring water level or something, then you have to learn protocol unit, which protocol you are going to use there, for example if water flow is there you gonna get message in the sense you need to learn some GSM module and Wifi protocol which is IEEE802.11g here the matter is you need to learn protocol unit to connect between devices (little networking helps more to understand how this protocol unit works ) then the heart of the project micro controller based on your need you have to choose micro controller, I suggest you can use Node MCU hardly cost you around 400 less than arduino uno, then piece of programming, you have to program your microcontroller to enable your protocol unit and sensor unit, and I advise you to concentrate on battery management to get it as product, these all are the things you have to learn before step in