How to control the output signal of this signal?
This amplifier will not be able to drive large 8 ohm speakers very loud. I conclude this from the two 5 ohm resistors present at the emitters of the output transistors. The amplifier will be sufficient to drive a small speaker like the ones most people use with their PC.

It will be able to drive three 8 ohm speakers in parallel but it will not be able to drive these very loud. Most power will be lost through the two 5 ohm resistors !

But why build your own amplifier from discrete components ? If you want the learning expercience then EXCELLENT :-) go ahead and build. If you want something a bit easier, there are ready-to-use modules on ebay like this class D type or this TDA2030 based one. These can only drive one speaker per amplifier but they are cheap. Also, these don't require the +/- 15 V but only a single 5 or 12 V supply.

Oh, and volume control is done like this:



This uses an LM386 audio amplifier, you can just replace it with the amplifier circuit from your question. Use a LOGARITHMIC potmeter (marked in red) to get nice volume control. For fun temporarily replace it with a standard LINEAIR potmeter and notice how the volume control is "crazy" ! :-) Why is that ? Because our ears are also sort of "logarthmic".