This, as most things, will firstly boil down to how much money you’re willing to spend. Prices for PA-systems will run from a few hundred Euros (or dollars) to tens of thousands.
Secondly, you’ll need to decide if you want an active or passive system. For reference, check the bundles Thomann offers.
Active sets: http://www.thomann.de/gb/active_pa_sets.html
Passive sets: http://www.thomann.de/gb/passive_pa_sets.html
Regarding connectivity, there’s really not that much difference in plugging into a PA or computer speakers – sound comes out of your controller via some type of cable, which then connects to an output device – a mixer/amp or, in the case of computer speakers, directly to the speakers which have a built-in amp.
Hope this helps get you started 🙂