legal requirements to open a service on the web, that requires payment

You follow the laws of Italy - but strictly speaking, you don't need to take care of any legal issues to start offering and selling services locally or internationally. Instead, you just need to respect your country's law on income taxes, as well as laws regulating the type of services you are offering.

For example, if you were planning to offer online Italian courses, you can setup your website and sell your services through PayPal or other payment gateway, all without setting up a company or even leaving your home.

However, you still need to report the money you earn on your income statement (or other relevant document in your country), and obey the laws of Italy which regulate educational services.

Regarding the company/society, as I said above, nobody is forcing you to have one if you obey all the laws regarding income tax/service provision. However, it is very practical to have a company if you plan on expanding your business - for example, you cannot cannot deduct business expenses from your income statement unless you have a company. To understand more about this, visit a webpage of Italy's government - there should be plenty information regarding this area.

Hope this helps.

Re: How can i determinate which country tax law i have to follow?

I'm not a lawyer, but:

Most western countries have double taxation avoidance agreements which mean that a business only has to pay corporate taxes in the country where it is located. So if you sell to the US and other countries with which Italy has a tax treaty, you will only have to pay corporate or business taxes in Italy. As far as incorporation, it's a good idea but it doesn't affect the above, you will not have to pay international taxes either way.