How do I fix the speed of a generator?

Electrically

Some systems do this electrically. The generator either generates DC, or the variable-frequency AC is rectified to make DC, and then an inverter makes the desired AC frequency. Common on more modern small wind turbines.

Mechanically

Other systems are mechanically controlled to get the desired frequency. The mechanism used would be called a governor. Most simple mechanical governors are not very accurate, so this would not be good enough for a grid connected device. It is also possible to make more accurate governors which work mechanically in a similar way to the paragraph below, these are commonly used on internal combustion engines.

With Feedback

Another approach, and probably the most common is to have some form of feedback. A microcontroller monitors the frequency being generated, and adjusts the mechanical system via some form of servo to get the right frequency. For example, it could open and close a sluice gate to adjust the water flow through a turbine. A more complicated system could adjust both a sluice gate and the turbine blades to keep the correct frequency while also varying the output power.

Grid Synchronous operation

In some cases, it might not be necessary at all. If you have a small wind turbine, connected to the mains grid near a coal power station, you could just hook it up and forget about it. The huge turbines in the power station will stabilise the grid frequency, and fix the rotation speed of the wind turbine. If the wind blows harder, you'll just get more current, and a slight power factor shift. Note that as more and more wind turbines get added, the folks who run the power station will get less and less happy about this, so the grid operator will eventually ban it.

If I need to generate power at a specified frequency, then I need to make sure that the rotor of the generator rotates at a specified speed (rpm).

No, that is not necessarily true. A lot of wind generators use a doubly fed induction generator (DFIG) and that can regulate frequency by controlling the rotor winding: -

They can convert power at one frequency to another frequency i.e. they can produce 50/60 Hz even though the rotor may be running too slowly. This is done by injection of an AC current into the rotor coils. The control system that acheives this may also be able to alter pitch angle of the turbine as another means of increasing or decreasing mechanical rotation speed.

For more information please read this EE answer.

But when I am rotating it with steam or water how do I control this speed?

There can be situations when there is too much rotational speed and although the DFIG can deal with this it is best to have a combined approach such as the pitch angle control shown in the above picture.

However, the inevitable bottom line is that if you have a plentiful source of mechanical energy and very little load demand at that moment, then you have to un-hook the generator from the grid. If you don't "have" a grid, then you have to have a back-up supply that can supply the low power demand and this usually means a diesel generator or solar power via an inverter.

That is what a governor is for.

The mechanical version is a device that will use centrifugal force or a blower to actuate the throttle/intake to slow down the engine when the speed climbs too much.

You can make it electronic with an RPM sensor and a electronically controlled throttle/intake.