Ask HN: I want to charge in my web application, what to do legally?

I am a non-US citizen and started creating a web application. I want to charge credit cards in the website, but I don't think it will bring big amounts of revenue - let's say $100 per month. I want to do this not for the money (although if possible, why not), but more for the sake of it. I have no idea what I should do legally. I want to charge internationally - having customers in Europe and the US. Should I incorporate? Does it matter where?

I'm asking it in the simplest form as I honestly don't know where to start here. I don't want to pay hundreds of dollars on legal/accounting counseling or hundreds of dollars for incorporation just for a small project that IF successful will only be worth a few hundred dollars for me. What do I need to do to legally charge and issue receipts worldwide online?

edit: To clarify, I have a paypal account (Premier, not business as I don't have a registered business), but the question is not about how technically to charge and process credit cards, but what do I need to do to make it fully compliant with the law (particularly American and EU law).