What is the quick way to speak louder, because my voice is very soft?
There is no “quick” way because you need to develop the strength the same as you would develop any kind of athletic strength. Otherwise you could injure yourself. You will need to support the voice by pacing the breath, getting a precise and adequate phonation and accessing the full resonance of your chest and head. That being said, if you speak in the same way as you would to make a nasty “a” sound, like in the word “cat” you will find some frontal resonance that should help add a little volume.