Hydroelectric energy relies on water and gravity. The water goes down a tube called the penstock, then the force of the water going downwards manages to spin a turbine. The kinetic energy from the spinning turbine then goes to a generator, which is connected to the tranformer. Then the energy travels to the power line which powers your home. Here is an image that explains everything.
You will find inside these places several giant generators... Well, in this room anyway. They are a lot bigger than you think. To make a hydro station, you have to start by building a dam, but not like a beaver dam, I mean a massive, concrete dam. Dams are usually curved for maximum strength. As you can see in the photo below, there are many buoys surrounding a large body of water. The reason why they put them there is to keep kayakers, divers etc. safe. The current from the water going through the tube is so strong that you will get sucked in!