The first thing that you have to do is get a Pa Package that comes with a powered mixer, cables, speakers, mics, ect
First place the powered mixer somewhere in your band set up thats out of the way, but easy to get to.
Now place the pa speakers either on the ground facing twards the band or mounted on the stands faceing twards the crowd.
Now hook up the cable that goes between the mixer and spaekers. There should be to of them. Try to keep them out of the way so you don't trip on them. Also you can tape the cable to the ground so it does not move.
Now hook up your mic to the mic cable and then hook the other end up to the mixer.
Now you can add other mics or instruments to the mixer and start rocking out.
Make sure that you have an extension chord because you may need one if your not near a power outlet