What are collection agencies like? How should one deal with them?

I’m poor and I can’t pay anything. I’ll spare you the sob story, and in exchange, please don’t pester me about responsibilities about payments and crap. I know only too well.
So I’ll just get to the question at hand.
Third party agencies attempt to try and collect the debt, I know that much. They’re regulated to many laws but are apparently still very painful to deal with. What can they or can they not do, and how to deal with them? Also, if I’m currently taking payment arrangements with the service providers themselves, (Not sure if they’re the first party or if that’s something else.) can they still sick collection agencies after you anyway? I need to know what to do in case they decide to. (I don’t know how any of this works so it’s confusing.)