Hey Dude, you don't want to ever plug the speaker cable from your amp into a mixing board. You will break something and you may hurt your self. This is because there is quite a lot of "amplified" juice running down you speaker cable and very little "amplified" juice running from say your guitar to the amp , or from the guitar to the stomp box to a desk. Or from the headphones out of your amp to a desk.

As for setting up a PA well, you will have to be a bit more specific about what kit you want to piece together and we will see what we can do!