How can I use microphone of laptop to output the voice…

You can do it, but you would have to place the laptop a good distance away so you don't get feedback. Most laptop speakers aren't that loud. How big is your audience?

 

You can go two ways: Both involve self powered speakers such as those for the ipod. Both also involve a wireless mic to prevent feedback. You can hook the speakers to the computer earphone jack or hook the mic straight to the speakers. You would basically end up with a small system like this: http://www.venturaes.com/index_new.asp?http://www.venturaes.com/califone/index3.html. Scroll down to the PA Pro 10. You could probably put one together for around $75-$100.