I am a big fan of hydro, but most people don't have water running through their property (I hope to when I retire soon).
The first thing is to have, or make (by building a flume), as much head as possible, then measure the volume of flow. Then the fun begins (be aware I haven't made anything yet, I've just read and recorded other peoples results).
If the head (amount of drop from input to outflow) of your system is only a couple of feet, you can spin a car alternator with a simple Banki turbine to get 12 volts (sadly, low amps)
Or, if you want to make a big water wheel, a Poncelet with curved buckets is a little more efficient than a simple wheel with flat paddles (like a river boat). Having just the bottom of a water wheel in the flow makes it an "undershot". A large diameter will turn slow, a small diameter will turn fast. Also, in order to add adequate torque to a fast spinning small diameter undershot, you may have to make it very wide (depending on the load you put on it).
from two to 10 feet of head, it is popular to have a hipshot/breastshot wheel. This is where the water flow is introduced to the wheel just above the axle. The weight of the water pulling down can be calculated (given adequate flow, of course) to determine how wide the buckets need to be to get enough torque to get the job done.
Above ten feet of head, you have the option of using a high-torque, low RPM "overshot" wheel if you want, but unless you're grinding grain, you may need gears to raise the RPM's if you want to generate electricity.
Water gains about 19 PSI for every 10 feet of head when it is contained in a pipe. From 50 feet to 100 feet, you might be interested in a high RPM "Pelton Wheel". It uses a small volume of water, but a high-pressure jet shoots at "spoons" around its rim that allow the water jet to flow around a curved path to extract as much energy as possible.
As you near 100 feet (or more) you might be able to use a water turbine, which looks like a turbocharger compressor. The flow can be precisely adjusted to give you an exact voltage at a very stable output, for direct use, or for charging a battery pack.
There are sooo many variables...what is your head height, flow rate, and desired work load?